add aux_flags field to HVs with aux struct

add aux_flags field to HVs with aux struct

Add an extra U32 general flags field to the xpvhv_aux struct (which is
used on HVs such as stashes, that need extra fields).

On 64-bit systems, this doesn't consume any extra space since there's
already an odd number of I32/U32 fields. On 32-bit systems it will consume
an  extra 4 bytes. But of course only on those hashes that have the aux
struct.

As well as providing extra flags in the AUX case, it will also allow
us to free up at least one general flag bit for HVs - see next commit.