On some platforms (currently ARM and ARM64), when calling into
VDSO we store the G to the gsignal stack, if there is one, so if
we receive a signal during VDSO we can find the G.

If we receive a signal during VDSO, and within the signal handler
we call nanotime again (e.g. when handling profiling signal),
we'll save/clear the G slot on the gsignal stack again, which
clobbers the original saved G. If we receive a second signal
during the same VDSO execution, we will fetch a nil G, which will
lead to bad things such as deadlock.

Don't save G if we're calling VDSO code from the gsignal stack.
Saving G is not necessary as we won't receive a nested signal.

Fixes #35473.

Change-Id: Ibfd8587a3c70c2f1533908b056e81b94d75d65a5
Reviewed-on: https://go-review.googlesource.com/c/go/+/206397
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>