runtime: clear m.gsignal when the M exits

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.

When an M exits, it frees the gsignal stack. But m.gsignal.stack
still points to that stack. When we call nanotime on this M, we
will write to the already freed gsignal stack, which is bad.
Prevent this by unlinking the freed stack from the M.

Should fix #35235.

Change-Id: I338b1fc8ec62aae036f38afaca3484687e11a40d
Reviewed-on: https://go-review.googlesource.com/c/go/+/204158
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/proc.go

@@ -1190,6 +1190,11 @@ func mexit(osStack bool) {
 	// Free the gsignal stack.
 	if m.gsignal != nil {
 		stackfree(m.gsignal.stack)
+		// On some platforms, when calling into VDSO (e.g. nanotime)
+		// we store our g on the gsignal stack, if there is one.
+		// Now the stack is freed, unlink it from the m, so we
+		// won't write to it when calling VDSO code.
+		m.gsignal = nil
 	}
 
 	// Remove m from allm.