runtime: use asmcgocall_no_g when calling sigprocmask on openbsd

sigprocmask is called from sigsave, which is called from needm. As such,
sigprocmask has to be able to run with no g. For some reason we do not
currently trip this on current libc platforms, but we do hit it on
openbsd/mips64 with external linking.

Updates #36435

Change-Id: I4dfae924245c5f68cc012755d6485939014898a3
Reviewed-on: https://go-review.googlesource.com/c/go/+/334879


Trust: Joel Sing <joel@sing.id.au>
Reviewed-by: Ian Lance Taylor <iant@golang.org>