internal/buildcfg: support GOPPC64=power10

internal/buildcfg: support GOPPC64=power10

This does not enable any new functionality. It should
behave identically to GOPPC64=power9.

Updates #44549

Change-Id: I9a860544527fcfe97cbaf89686459d40dcf9593e
Reviewed-on: https://go-review.googlesource.com/c/go/+/352791
Run-TryBot: Paul Murphy <murp@ibm.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>