runtime: remove type-punning for Type.gc[0], gc[1]

Depending on flags&KindGCProg,
gc[0] and gc[1] are either pointers or inlined bitmap bits.
That's not compatible with a precise garbage collector:
it needs to be always pointers or never pointers.

Change the inlined bitmap case to store a pointer to an
out-of-line bitmap in gc[0]. The out-of-line bitmaps are
dedup'ed, so that for example all pointer types share the
same out-of-line bitmap.

Fixes #8864.

LGTM=r
R=golang-codereviews, dvyukov, r
CC=golang-codereviews, iant, khr, rlh
https://golang.org/cl/155820043

src/reflect/type.go

@@ -1523,8 +1523,8 @@ func (gc *gcProg) appendProg(t *rtype) {
 		// The program is stored in t.gc[0], skip unroll flag.
 		prog = (*[1 << 30]byte)(unsafe.Pointer(t.gc[0]))[1:]
 	} else {
-		// The mask is embed directly in t.gc.
-		prog = (*[1 << 30]byte)(unsafe.Pointer(&t.gc[0]))[:]
+		// The mask is linked directly in t.gc.
+		prog = (*[2 * ptrSize]byte)(unsafe.Pointer(t.gc[0]))[:]
 	}
 	for i := uintptr(0); i < nptr; i++ {
 		gc.appendWord(extractGCWord(prog, i))