cmd/compile: extend GOSSAFUNC match to well-formed package suffix match.
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 <ae171625adc8645b8b79b7e405a6da5aba5341e0@google.com>
Run-TryBot: David Chase <ae171625adc8645b8b79b7e405a6da5aba5341e0@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>

cloudflare/go,CAFxX/go,nawawi/go,CAFxX/go,golang/go,cloudflare/go,CAFxX/go,cloudflare/go,nawawi/go,cloudflare/go,nawawi/go,CAFxX/go,cloudflare/go,cloudflare/go,nawawi/go,golang/go,golang/go,cloudflare/go,CAFxX/go,nawawi/go,nawawi/go,golang/go,golang/go,golang/go,CAFxX/go,CAFxX/go,golang/go,cloudflare/go,golang/go,nawawi/go,CAFxX/go,nawawi/go