runtime: fix type cast in assignment to gcnextsp in C code

Patch from Rainer Orth.

Change-Id: I39d094aeb0063e1b9c08dcc20404ee41fe0c741f
Reviewed-on: https://go-review.googlesource.com/46459
Reviewed-by: Ian Lance Taylor <iant@golang.org>
diff --git a/libgo/runtime/proc.c b/libgo/runtime/proc.c
index 303a1b5..1272669 100644
--- a/libgo/runtime/proc.c
+++ b/libgo/runtime/proc.c
@@ -678,7 +678,7 @@
 	{
 		void *v;
 
-		g->gcnextsp = (byte *) &v;
+		g->gcnextsp = (uintptr)(&v);
 	}
 #endif