runtime: cast SIGSTKSZ to uintptr

In newer versions of glibc it is long, which causes a signed
comparison warning.

Fixes https://gcc.gnu.org/PR99458

Change-Id: I9fca4a971455a1361f5435d0ecb80e1a5096486c
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/299849
Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
diff --git a/libgo/runtime/proc.c b/libgo/runtime/proc.c
index c037df6..38bf7a6 100644
--- a/libgo/runtime/proc.c
+++ b/libgo/runtime/proc.c
@@ -802,8 +802,8 @@
 		if(signalstack) {
 			stacksize = 32 * 1024; // OS X wants >= 8K, GNU/Linux >= 2K
 #ifdef SIGSTKSZ
-			if(stacksize < SIGSTKSZ)
-				stacksize = SIGSTKSZ;
+			if(stacksize < (uintptr)(SIGSTKSZ))
+				stacksize = (uintptr)(SIGSTKSZ);
 #endif
 		}