libatomic: Add support for LSE and LSE2

Add support for AArch64 LSE and LSE2 to libatomic.  Disable outline atomics,
and use LSE ifuncs for 1-8 byte atomics and LSE2 ifuncs for 16-byte atomics.
On Neoverse V1, 16-byte atomics are ~4x faster due to avoiding locks.

Note this is safe since we swap all 16-byte atomics using the same ifunc,
so they either use locks or LSE2 atomics, but never a mix. This also improves
ABI compatibility with LLVM: its inlined 16-byte atomics are compatible with
the new libatomic if LSE2 is supported.

libatomic/
	* Makefile.in: Regenerated with automake 1.15.1.
	* Makefile.am: Add atomic_16.S for AArch64.
	* configure.tgt: Disable outline atomics in AArch64 build.
	* config/linux/aarch64/atomic_16.S: New file - implementation of
	ifuncs for 16-byte atomics.
	* config/linux/aarch64/host-config.h: Enable ifuncs, use LSE
	(HWCAP_ATOMICS) for 1-8-byte atomics and LSE2 (HWCAP_USCAT) for
	16-byte atomics.