cmd/link: enable internal linker in more cases for ppc64le

The internal linker is capable of linking the ppc64le linux
race detector and approved cgo packages.

Likewise, ppc64/linux and ppc64/aix do not support the race
detector. Thus, extra code to enforce external linking when
using the race detector on ppc64/ppc64le can be removed
entirely.

Fixes #21961

Change-Id: I10db14f65ee616ee3291e17409e8333e3af7d4df
Reviewed-on: https://go-review.googlesource.com/c/go/+/304459


Run-TryBot: Paul Murphy <murp@ibm.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>