[dev.regabi] cmd/compile,cmd/link: additional code review suggestions for CL 270863

This patch pulls in a few additional changes requested by code
reviewers for CL 270863 that were accidentally left out. Specifically,
guarding use of ORETJMP to insure it is not used when building dynlink
on ppc64le, and a tweaking the command line flags used to control
wrapper generation.

Change-Id: I4f96462e570180887eb8693e11badd83d142710a
Reviewed-on: https://go-review.googlesource.com/c/go/+/279527


Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Trust: Than McIntosh <thanm@google.com>