cmd/link: check for power10 support on external linker for PPC64
The external linker will need to support the new PC relative relocations
when they are generated by Go in a future patch. If it does not, many
unhelpful relocation errors will be generated by the external linker.
Use the -mcpu=power10 option as a surrogate for -mpcrel. It most cases,
it should indicate whether the underlying linker has support for
resolving PC relative relocations.
Change-Id: I84b151ce04512ccaeb17835aaf44105a5f6b515b
Reviewed-on: https://go-review.googlesource.com/c/go/+/469576
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Paul Murphy <murp@ibm.com> Reviewed-by: Archana Ravindar <aravind5@in.ibm.com> Reviewed-by: Than McIntosh <thanm@google.com> Reviewed-by: Cherry Mui <cherryyz@google.com>