linux/ppc64 uses the ppc64 v1 ABI which was never fully supported
by Go. (linux/ppc64le uses the ppc64 v2 ABI and that is fully
supported).

As a result if the external linker is used to build a program
on ppc64, there is a either a warning or error message that doesn't
clearly describe the problem. In the case of a warning,
a program is created that will most likely not execute since it is not
built as expected for the ppc64 dynamic linker (ld64.so.1).

To avoid confusion in these cases, error messages are now issued
if external linker is explicitly used to build the program. Note that most
buildmodes that require external linking were already flagging linux/ppc64
as unsupported except for c-archive, which has been added here.

This problem does not occur with gccgo since the ppc64 v1 ABI is
supported there.

Fixes #25079

Change-Id: I44d77a1eb9df750d499cd432b0ca4a97f0be88b2
Reviewed-on: https://go-review.googlesource.com/109915Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>