This interns the asm flags parameter in the state struct
so we don't need to pass it with every call. It can be
expensive, so we don't want to query every run_analysis()
call, but since this (private) api is used by webrtc code
we need to provide a supportable interface for filling in
the correct value.

Note the initialization code is partially duplicated between
opus_encoder_init and the OPUS_RESET_STATE switch case, so we
must re-initialize it there.

Signed-off-by: Jean-Marc Valin <jmvalin@jmvalin.ca>