cmd/vet: fix go vet on parentheses of assembly function flag

cmd/vet: fix go vet on parentheses of assembly function flag

Current implementation doesn't recognize parentheses that may appear in flags
of assembly function as shown below:

	TEXT ·makeFuncStub(SB),(NOSPLIT|WRAPPER),$24

It results in vet reporting false positives and a lot of whitelists are added
for suppressing the false alarms.

This CL fixes the issue and eliminates the redundant whitelists.

Change-Id: Idbc1b42965b31cea8ee7c23d1a6f62feb68e844c
Reviewed-on: https://go-review.googlesource.com/62850
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>