libgo: change mksigtab to recognize glibc 2.26 NSIG expression

Fixes golang/go#21147
Fixes GCC PR 81617

Change-Id: I3d63704a45530faea28ab18278550c9b923f434f
Reviewed-on: https://go-review.googlesource.com/52611
Reviewed-by: Than McIntosh <thanm@google.com>
1 file changed