runtime: correct mips64 asmcgocall signal stack behaviour

Do not switch to the g0 stack if we're already running on the signal
stack, otherwise all kind of fun ensues.

Updates #36435

Change-Id: I57f35d75b9ee4f92b997713b4cdd38ce881705e6
Reviewed-on: https://go-review.googlesource.com/c/go/+/334880


Trust: Joel Sing <joel@sing.id.au>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>