gas: aarch64: Accept `purecap` and `hybrid` ABI parameters

gas: aarch64: Accept `purecap` and `hybrid` ABI parameters

When GCC is given an ABI parameter with `-mabi=<whatever>` it passes
that argument down to GAS.  GAS does not need to know the Morello ABI
that is being used, since all decisions are based on the processor state
(whether +c64 is enabled or not).

GAS doesn't currently accept `purecap` or `hybrid` as arguments to the
`-mabi` option.  Even though it does not need this information, I think
it should accept the arguments.  This would mean GCC does not need
implement special handling to avoid passing the `-mabi` argument to GAS
in these specific cases.

gas/ChangeLog:

2021-07-30  Matthew Malcomson  <matthew.malcomson@arm.com>

	* config/tc-aarch64.c (aarch64_abi_type): Introduce PURECAP and
	HYBRID enum entries.
	(aarch64_abis): Add "purecap" and "hybrid" parameters.
	* testsuite/gas/aarch64/morello-abis-ignored.s: New.
	* testsuite/gas/aarch64/morello-abis-ignored.d: New.