aarch64: Add arch support for LSE128 extension

Enable the `+lse128' feature modifier which, together with new
internal feature flags, enables LSE128 instructions, which are
represented via the new `_LSE128_INSN' macro.

gas/ChangeLog:

	* config/tc-aarch64.c (aarch64_features): Add new "lse128"
	entry.

include/ChangeLog:

	* include/opcode/aarch64.h (enum aarch64_feature_bit): New
	AARCH64_FEATURE_LSE128 feature bit.
	(enum aarch64_insn_class): New lse128_atomic instruction class.

opcodes/ChangeLog:

	* opcodes/aarch64-tbl.h (aarch64_feature_lse128): New.
	(LSE128): Likewise.
	(_LSE128_INSN): Likewise.