[AArch64, ILP32] Retire -milp32 and -mlp64; use -mabi=ilp32 and -mabi=lp64.

gas/

	* config/tc-aarch64.c (enum aarch64_abi_type): New enumeration tag.
	(AARCH64_ABI_LP64, AARCH64_ABI_ILP32): New enumerators.
	(aarch64_abi): New variable.
	(ilp32_p): Change to be a macro.
	(aarch64_opts): Remove the support for option -milp32 and -mlp64.
	(struct aarch64_option_abi_value_table): New struct.
	(aarch64_abis): New table.
	(aarch64_parse_abi): New function.
	(aarch64_long_opts): Add entry for -mabi=.
	* doc/as.texinfo (Target AArch64 options): Document -mabi.
	* doc/c-aarch64.texi: Likewise.

gas/testsuite/

	* gas/aarch64/ilp32-basic.d (#as): Update to use -mabi=ilp32