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

    
    Patch from Rainer Orth.
    
    Reviewed-on: https://go-review.googlesource.com/46459

From-SVN: r249575

gcc/go/gofrontend/MERGE

-681c8a7b0a9d52c0b81e7a4b1c55fe65ed889573
+27204e41643f4549c5ed97d48e17912f64c0aa71
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/runtime/proc.c

@@ -678,7 +678,7 @@ doentersyscallblock(uintptr pc, uintptr sp)
 	{
 		void *v;
 
-		g->gcnextsp = (byte *) &v;
+		g->gcnextsp = (uintptr)(&v);
 	}
 #endif