liblink, cmd/gc: resolve several shift warnings

Address several warnings generated by clang -fsanitize=undefined

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/43050043

src/cmd/gc/bv.c

@@ -98,7 +98,7 @@ bvget(Bvec *bv, int32 i)
 
 	if(i < 0 || i >= bv->n)
 		fatal("bvget: index %d is out of bounds with length %d\n", i, bv->n);
-	mask = 1 << (i % WORDBITS);
+	mask = 1U << (i % WORDBITS);
 	word = bv->b[i / WORDBITS] & mask;
 	return word ? 1 : 0;
 }

src/liblink/objfile.c

@@ -371,7 +371,7 @@ wrint(Biobuf *b, int64 sval)
 	uint64 uv, v;
 	uchar buf[10], *p;
 
-	uv = (uint64)(sval<<1) ^ (uint64)(int64)(sval>>63);
+	uv = ((uint64)sval<<1) ^ (uint64)(int64)(sval>>63);
 
 	p = buf;
 	for(v = uv; v >= 0x80; v >>= 7)
@@ -634,7 +634,7 @@ rdint(Biobuf *f)
 			break;
 	}
 
-	return (int64)(uv>>1) ^ ((int64)uv<<63>>63);
+	return (int64)(uv>>1) ^ ((int64)((uint64)uv<<63)>>63);
 }
 
 static char*

src/liblink/pcln.c

@@ -313,7 +313,7 @@ getvarint(uchar **pp)
 	v = 0;
 	p = *pp;
 	for(shift = 0;; shift += 7) {
-		v |= (*p & 0x7F) << shift;
+		v |= (uint32)(*p & 0x7F) << shift;
 		if(!(*p++ & 0x80))
 			break;
 	}