cmd/compile: extend GOSSAFUNC match to well-formed package suffix match.

e.g., LeadingZeros, bits.LeadingZeros, math/bits.LeadingZeros
but not its.LeadingZeros

Change-Id: Ib9a57e4db0af03c55bf5b5027aa6f8a5a84f2134
Reviewed-on: https://go-review.googlesource.com/c/go/+/312291


Trust: David Chase <drchase@google.com>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>