It's now easy to build a cross toolchain in Funtoo.  The steps are:

1.  emerge crossdev
2.  setup package.env to pass your target device CFLAGS to gcc
3.  setup crossdev CROSS-OVERLAY
4.  build your cross toolchain

1.  emerge crossdev
To use crossdev to create cross toolchains you must have crossdev.  As of this writing
the latest one in the Funtoo tree, crossdev-20180818 works fine.  

2.  setup package.env to pass your target device CFLAGS to gcc

The following steps must only be done once for each cross toolchain you wish to make.

Set up package.env to pass the target's CFLAGS to gcc.  I'll use my armv7a
odroid as an example.

First, I copy my CFLAGS from my armv7a device's /etc/portage/make.conf:

# ssh odroid grep CFLAGS /etc/portage/make.conf

Copy those flags and echo them to a file in /etc/portage/env/<your choice of name>:

# echo 'CFLAGS="-Ofast -march=armv7-a -mtune=cortex-a5 -mfpu=neon-vfpv4 -mfloat-abi=hard"' \
>> /etc/portage/env/cross-armv7a-hardfloat-linux-gnueabi-gcc.conf

Now reference that file in /etc/portage/package.env.  Crossdev will demand that 
/etc/portage/package.env be a directory, so if it's not already set up that way, now's the
time to do it.  Incidentally, crossdev also requires /etc/portage/package.mask and
/etc/portage/package.use to be directories.  Now's the time if they aren't.  I have a
/etc/portage/package.env/package.env file, crossdev will later place 
/etc/portage/package.env/<YOUR TUPLE> file(s) in there.
Leave those alone, as crossdev itself creates, updates, etc. those file(s).

# echo "cross-armv7a-hardfloat-linux-gnueabi/gcc  cross-armv7a-hardfloat-linux-gnueabi-gcc.conf" \
>> /etc/portage/package.env/package.env

3.  setup crossdev CROSS-OVERLAY

Crossdev uses the`portageq repositories_configuration` command to search for toolchain packages,
so it will find meta-repo and any other repos you have noted in /etc/portage/repos.conf.
However it will choose to put it's links to them in meta-repo/nokit.  This is probably not the
best place for this, so it's desirable to use your local repo instead by using -oO or
--ov-output <your local repo path> instead.  That way it won't interfere in any way or
get nuked if you rebuild meta-repo.
Initialize your CROSS-OVERLAY thusly:
# crossdev -t <your tuple> --init-target -oS <path to local repo>
In my case:
# crossdev -t armv7a-hardfloat-linux-gnueabi --init-target -oS /var/git/borg-repo

Currently the only gcc ebuilds capable of building a cross gcc are 6.4.1-r4, 7.3.1-r4 and
8.2.1-r4.  As these ebuild propagate to the tree and future ebuilds will have the capability
to build cross gcc the following information will no longer be necessary.
After you have initialized your CROSS-OVERLAY in <path to local repo>/cross-<tuple> you will find
links similar to this:
# ls -l <path to local repo>/cross-<tuple>
(or in my case: # ls -l /var/git/borg-repo/cross-armv7a-hardfloat-linux-gnueabi)

lrwxrwxrwx 1 root root 51 Sep 14 17:31 binutils -> /var/git/meta-repo/kits/core-kit/sys-devel/binutils
lrwxrwxrwx 1 root root 34 Sep 14 19:04 gcc -> /var/git/meta-repo/kits/core-kit/sys-devel/gcc
lrwxrwxrwx 1 root root 43 Sep 14 17:31 gdb -> /var/git/meta-repo/kits/nokit/sys-devel/gdb
lrwxrwxrwx 1 root root 47 Sep 14 17:31 glibc -> /var/git/meta-repo/kits/core-kit/sys-libs/glibc
lrwxrwxrwx 1 root root 57 Sep 14 17:31 linux-headers -> /var/git/meta-repo/kits/core-kit/sys-kernel/linux-headers

You must delete the gcc link and replace it with a link to your local repo gcc builds:

# cd <path to local repo>/cross-<tuple>
# rm gcc
# ln -s <path to local repo>/sys-devel/gcc gcc

Otherwise crossdev will insist on using Funtoo tree ebuilds, even bumping the version to match them
if necessary.

4.  build your cross toolchain
Now you can run crossdev to create your toolchain.  If you've already built a cross toolchain with
gcc remove the previous gcc version with emerge --unmerge =TUPLE/gcc-VERSION first.  Be sure to use -oS
or --overlays to point crossdev to the CROSS-OVERLAY.  Here's an example I ran successfully today:

# crossdev --b 2.29.1-r1 --g 6.4.1-r4 --k 4.9 --l 2.23-r4 -P -v -t armv7a-hardfloat-linux-gnueabi -oS /var/git/borg-repo

This handy script run on the machine you are making the cross toolchain(s) for takes the work out of figuring it out:
--------------------
#!/bin/bash
BINUTILS_VER=$(equery l sys-devel/binutils|cut -d- -f3-)
GCC_VER=$(gcc --version|grep -A1 Funtoo|cut -d ' ' -f3|cut -d ')' -f1|head -n1)
KERNEL_VER=$(equery l sys-kernel/linux-headers|cut -d- -f4-)
LIBC_VER=$(equery l sys-libs/glibc|cut -d- -f3-)
echo "crossdev --b $BINUTILS_VER --g $GCC_VER --k $KERNEL_VER --l $LIBC_VER -P -v -t $(portageq envvar CHOST) -oS <your local repo path> "
--------------------

Note that on cross gcc ebuilds you will get file collision warnings at the end of install,
not a cause for alarm.  These are config (such as /etc/env.d) files that are being replaced.
Portage should tell you they are going to be installed in spite of the collisions when it finishes.
Don't forget to run source /etc/profile and gcc-config when it's done, as it's not automatically
enabled.

Addendum:
On 9-17-18 I executed the following command:
crossdev --b 2.29.1-r1 --g 6.4.1-r4 --k 4.14 --l 2.26-r6 -P -v -t armv7a-hardfloat-linux-gnueabi -oS /var/git/borg-repo
Glibc failed, complaining that I needed to use the -fPIC directive.  Since glibc does not have a fPIC use flag I
created another /etc/package.env file:
# cat /etc/portage/env/cross-armv7a-hardfloat-linux-gnueabi-glibc.conf
CFLAGS="${CFLAGS} -fPIC"
And added this to /etc/portage/package.env/package.env:
cross-armv7*/glibc			cross-armv7a-hardfloat-linux-gnueabi-glibc.conf
And the toolchain emerged successfully.