runtime: don't fetch G from signal stack when using cgo

When using cgo, we save G to TLS, and when a signal happens, we
load G from TLS in sigtramp. This should give us a valid G. Don't
try to fetch from the signal stack. In particular, C code may
change the signal stack or call our signal handler directly (e.g.
TSAN), so we are not necessarily running on the original gsignal
stack where we saved G.

Also skip saving G on the signal stack when using cgo.

Updates #35249.

Change-Id: I40749ce6682709bd4ebfdfd9f23bd0f317fc197d
Reviewed-on: https://go-review.googlesource.com/c/go/+/204519


Reviewed-by: Ian Lance Taylor <iant@golang.org>