[AArch64, gas] Support ILP32 triplet aarch64*-linux-gnu_ilp32

[AArch64, gas] Support ILP32 triplet aarch64*-linux-gnu_ilp32

This patch allows AArch64 GAS defaulting to ILP32 if it is configured with
aarch64*-linux-gnu_ilp32.

"md_after_parse_args" is implemented to update ABI into ILP32 if DEFAULT_ARCH is
"aarch64:32".

gas/
	* configure.tgt: Set "arch" to "aarch64" if ${cpu} equals "aarch64".
	Recognize the new triplet name aarch64*-linux-gnu_ilp32.
	* configure.ac: Output DEFAULT_ARCH macro for AArch64.
	* configure: Regenerate.
	* config/tc-aarch64.h (aarch64_after_parse_args): New declaration.
	(md_after_parse_args): New define.
	* config/tc-aarch64.c (aarch64_abi_type): New enumeration
	AARCH64_ABI_NONE.
	(DEFAULT_ARCH): New define.
	(aarch64_abi): Set default value to AARCH64_ABI_NONE.
	(aarch64_after_parse_args): New function.