runtime: use C cast syntax in stack.c

Didn't notice earlier because this code is only used on systems that
do not support -fsplit-stack.

Change-Id: I101f91b67a09526867c940fe33a0778d0f7f6be4
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/342051
Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>

libgo/runtime/stack.c

diff --git a/libgo/runtime/stack.c b/libgo/runtime/stack.c
index 0c0c30e..65e8fa3 100644
--- a/libgo/runtime/stack.c
+++ b/libgo/runtime/stack.c
@@ -174,7 +174,7 @@
 	nextsp2 = secondary_stack_pointer();
 	if (nextsp2 != nil) {
 		initialsp2 = (byte*)(void*)(gp->gcinitialsp2);
-		if (uintptr(initialsp2) < uintptr(nextsp2)) {
+		if ((uintptr)(initialsp2) < (uintptr)(nextsp2)) {
 			temp = initialsp2;
 			initialsp2 = nextsp2;
 			nextsp2 = temp;