runtime: mlock signal stack on macOS/ARM64

Apparently, the macOS ARM64 kernel has a bug where when a signal
arrives and the signal stack is not currently faulted in, it may
kill the program with a SIGILL. Work around it by mlock the
signal stacks.

Fixes #42774.

Change-Id: I99a4b3fdb6d8af1c945725ddc2c25568d81c510a
Reviewed-on: https://go-review.googlesource.com/c/go/+/273686


Trust: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Go Bot <gobot@golang.org>