cmd/link: implement external linking support for ppc64/linux

[ianlancetaylor]

We need to fix the 9l linker to provide external linking support, so that
-linkmode=external works on power64.

This code is currently on the dev.power64 branch and should come into tip after the 1.4
release is out.  This means writing elfreloc1, elfsetupplt, adddynsym, adddynrela, etc.,
in cmd/9l/asm.c and doing whatever else is necessary so that (cd misc/cgo/test
&& go test -linkmode=external) passes for both big- and little- endian power64.

[ianlancetaylor]

Comment 2:

Doing "whatever else is necessary" to make misc/cgo/test pass will include writing
asm_power64be.s, asm_power64le.s, and gcc_linux_power64{be,ls}.s in the runtime/cgo
package.

[gopherbot]

Comment 5 by rappo@bountysource.com:

There is a $1,200 bounty on this issue from IBM:
https://www.bountysource.com/issues/5259148-cmd-9l-implement-external-linking-support-for-power64-linux

[minux]

Comment 6:

Issue #9038 has been merged into this issue.