reflect: generated unrolled GC bitmask directly

The code for a generated type is already generating an
unrolled GC bitmask.  Rather than unrolling the the source
type bitmasks and copying them, just generate the required
bitmask directly.  Don't mark it as an unrolled GC program,
since there is no need to do so.

Fixes issue 8917.

LGTM=rsc
R=dvyukov, rsc
CC=golang-codereviews
https://codereview.appspot.com/156930044