Only regenerate tlsoffsets{64}.h if cygtls.h can be compiled

Only regenerate tlsoffsets{64}.h if cygtls.h can be compiled

The rule to make tlsoffset{64}.h has a flaw.  If cygtls.h can't be
built for whatever reason, it *still* regenerates tlsoffsets{64}.h,
just with size 0.  If the bug is not in cygtls.h itself, this behaviour
breaks further building, because fixing the problem won't result in
regenerating tlsoffset{64}.h.  Manual intervention is required.

Fix that by removing tlsoffsets{64}.h if gentls_offsets fails.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>