cmd/link: don't use internal linking mode for cgo on PPC64
The internal linker doesn't know how to handle multiple TOC sections
in internal linking mode. This used to work because before CL 64793 we
invoked ld -r on multiple objects, and that merged the TOC sections
for us.
Updates #21961
Change-Id: I48260a7195be660016f2f358ebc8cb79652210ab
Reviewed-on: https://go-review.googlesource.com/66270
Run-TryBot: Ian Lance Taylor <iant@golang.org> Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>