runtime: correct mips64 asmcgocall signal stack behaviour
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 <4816dabf8db1bc6cac35b3a24cab2ff844b5b0c7@sing.id.au>
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Cherry Mui <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>

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