runtime: fix plan9/amd64 build?

The only thing I can see that is really Plan 9-specific
is that the stack pointer used for signal handling used
to have more mapped memory above it.
Specifically it used to have at most 88 bytes (StackTop),
so change the allocation of a 40-byte frame to a 128-byte frame.

No idea if this will work, but worth a try.

Note that "fix" here means get it back to timing out
instead of crashing.

TBR=iant
CC=golang-codereviews
https://golang.org/cl/142840043

src/runtime/sys_plan9_amd64.s

@@ -194,7 +194,7 @@ TEXT runtime·sigtramp(SB),NOSPLIT,$0
 	MOVQ	BP, SP
 
 	// make room for args and g
-	SUBQ	$40, SP
+	SUBQ	$128, SP
 
 	// save g
 	MOVQ	g(AX), BP