cmd/ld: fix undefined behaviour in pcln.c

Update #8527

Fixes, src/cmd/6l/../ld/pcln.c:93:18: runtime error: left shift of negative value -2

LGTM=r
R=golang-codereviews, r
CC=golang-codereviews
https://golang.org/cl/127440043

src/cmd/ld/pcln.c

@@ -90,7 +90,7 @@ renumberfiles(Link *ctxt, LSym **files, int nfiles, Pcdata *d)
 		}
 		dv = val - newval;
 		newval = val;
-		v = (uint32)(dv<<1) ^ (uint32)(int32)(dv>>31);
+		v = ((uint32)dv<<1) ^ (uint32)(int32)(dv>>31);
 		addvarint(&out, v);
 
 		// pc delta