runtime/cgo: use frame address to set g0 stack bound

For a cgo binary, at startup we set g0's stack bounds using the
address of a local variable (&size) in a C function x_cgo_init and
the stack size from pthread_attr_getstacksize. Normally, &size is
an address within the current stack frame. However, when  it is
compiled with ASAN, it may be instrumented to __asan_stack_malloc_0
and the address may not live in the current stack frame, causing
the stack bound to be set incorrectly, e.g. lo > hi.

Using __builtin_frame_address(0) to get the stack address instead.

Change-Id: I41df929e5ed24d8bbf3e15027af6dcdfc3736e37
Reviewed-on: https://go-review.googlesource.com/c/go/+/419434
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/runtime/cgo/gcc_linux_amd64.c

@@ -44,7 +44,9 @@ x_cgo_init(G *g, void (*setg)(void*), void **tlsg, void **tlsbase)
 	}
 	pthread_attr_init(attr);
 	pthread_attr_getstacksize(attr, &size);
-	g->stacklo = (uintptr)&size - size + 4096;
+	g->stacklo = (uintptr)__builtin_frame_address(0) - size + 4096;
+	if (g->stacklo >= g->stackhi)
+		fatalf("bad stack bounds: lo=%p hi=%p\n", g->stacklo, g->stackhi);
 	pthread_attr_destroy(attr);
 	free(attr);