runtime: guard against runtime/sema* ops on Darwin signal stack.

runtime: guard against runtime/sema* ops on Darwin signal stack.

These operations misbehave and cause hangs and flakes.
Fail hard if they are attempted.

Tested by backing out the Darwin-profiling-hang fix
CL 518836 and running run.bash, the guard panicked in
runtime/pprof tests, as expected/hoped.

Updates #61768

Change-Id: I89b6f85745fbaa2245141ea98f584afc5d6b133e
Reviewed-on: https://go-review.googlesource.com/c/go/+/519275
Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>