aarch64: Move ZA range checks to aarch64-opc.c

aarch64: Move ZA range checks to aarch64-opc.c

This patch moves the range checks on ZA vector select offsets from
gas to libopcodes.  Doing the checks there means that the error
messages contain the expected range.  It also fits in better
with the error severity scheme, which becomes important later.
(This is because out-of-range indices are treated as more severe than
syntax errors, on the basis that parsing must have succeeded if we get
to the point of checking the completed opcode.)

The patch also adds a new check_za_access function for checking
ZA accesses.  That's a bit over the top for one offset check, but the
function becomes more complex with later patches.

sme-9-illegal.s checked for an invalid .q suffix using:

  psel p1, p15, p3.q[w15]

but this is doubly invalid because it misses the immediate part
of the index.  The patch keeps that test but adds another with
a zero index, so that .q is the only thing wrong.

The aarch64-tbl.h change includes neatening up the backslash
positions.