cmd/compile, runtime: stop padding stackmaps to 4 bytes

Shrinks cmd/go's text segment by 0.9%.

   text	   data	    bss	    dec	    hex	filename
6447148	 231643	 146328	6825119	 68249f	go.before
6387404	 231643	 146328	6765375	 673b3f	go.after

Change-Id: I431e8482dbb11a7c1c77f2196cada43d5dad2981
Reviewed-on: https://go-review.googlesource.com/30817Reviewed-by: Austin Clements <austin@google.com>

src/cmd/compile/internal/gc/obj.go

@@ -272,14 +272,10 @@ func duintptr(s *Sym, off int, v uint64) int {
 }
 
 func dbvec(s *Sym, off int, bv bvec) int {
-	for j := 0; int32(j) < bv.n; j += 32 {
+	// Runtime reads the bitmaps as byte arrays. Oblige.
+	for j := 0; int32(j) < bv.n; j += 8 {
 		word := bv.b[j/32]
-
-		// Runtime reads the bitmaps as byte arrays. Oblige.
-		off = duint8(s, off, uint8(word))
-		off = duint8(s, off, uint8(word>>8))
-		off = duint8(s, off, uint8(word>>16))
-		off = duint8(s, off, uint8(word>>24))
+		off = duint8(s, off, uint8(word>>(uint(j)%32)))
 	}
 	return off
 }

src/runtime/symtab.go

@@ -603,5 +603,5 @@ func stackmapdata(stkmap *stackmap, n int32) bitvector {
 	if n < 0 || n >= stkmap.n {
 		throw("stackmapdata: index out of range")
 	}
-	return bitvector{stkmap.nbit, (*byte)(add(unsafe.Pointer(&stkmap.bytedata), uintptr(n*((stkmap.nbit+31)/32*4))))}
+	return bitvector{stkmap.nbit, (*byte)(add(unsafe.Pointer(&stkmap.bytedata), uintptr(n*((stkmap.nbit+7)/8))))}
 }