install_db4.sh configure cannot guess build type on ppc64le

[JeremyRand]

Describe the issue

The configure script called by install_db4.sh fails to detect the build arch on ppc64le systems, producing an error.

What behavior did you expect?

db4 should build properly without errors.

What was the actual behavior (provide screenshots if the issue is GUI-related)?

checking build system type... /home/user/Downloads/bitcoin/db4/db-4.8.30.NC/dist/config.guess: unable to guess system type

This script, last modified 2009-02-03, has failed to recognize
the operating system you are using. It is advised that you
download the most up to date version of the config scripts from

  http://git.savannah.gnu.org/gitweb/?p=config.git;a=blob_plain;f=config.guess;hb=HEAD
and
  http://git.savannah.gnu.org/gitweb/?p=config.git;a=blob_plain;f=config.sub;hb=HEAD

If the version you run (/home/user/Downloads/bitcoin/db4/db-4.8.30.NC/dist/config.guess) is already up to date, please
send the following data and any information you think might be
pertinent to <config-patches@gnu.org> in order to provide the needed
information to handle your system.

config.guess timestamp = 2009-02-03

uname -m = ppc64le
uname -r = 4.19.0-5-powerpc64le
uname -s = Linux
uname -v = #1 SMP Debian 4.19.37-3 (2019-05-15)

/usr/bin/uname -p = 
/bin/uname -X     = 

hostinfo               = 
/bin/universe          = 
/usr/bin/arch -k       = 
/bin/arch              = 
/usr/bin/oslevel       = 
/usr/convex/getsysinfo = 

UNAME_MACHINE = ppc64le
UNAME_RELEASE = 4.19.0-5-powerpc64le
UNAME_SYSTEM  = Linux
UNAME_VERSION = #1 SMP Debian 4.19.37-3 (2019-05-15)
configure: error: cannot guess build type; you must specify one

How reliably can you reproduce the issue, what are the steps to do so?

1. ./contrib/install_db4.sh `pwd`

What version of Bitcoin Core are you using, where did you get it (website, self-compiled, etc)?

2d1583e , self-compiled.

What type of machine are you observing the error on (OS/CPU and disk type)?

In a VM running Whonix 15 (based on Debian Buster); host is Debian Buster. ppc64le / POWER9. HDD.

Any extra information that might be useful in the debugging process.

The following does work: ./contrib/install_db4.sh `pwd` --build=powerpc64le

While it would be nice to fix the configure script to properly detect powerpc64le, I'd settle for a note in build-unix.md that specifies the above workaround so that newbies don't have to figure out the configure flag on their own.

[fanquake]

@JeremyRand Can you test #16086 ?