Libatomic: Clean up AArch64 `atomic_16.S' implementation file

At present, `atomic_16.S' groups different implementations of the
same functions together in the file.  Therefore, as an example,
the LSE2 implementation of `load_16' follows on immediately from its
core implementation, as does the `store_16' LSE2 implementation.

Such architectural extension-dependent implementations are dependent
on ifunc support, such that they are guarded by the relevant
preprocessor macro, i.e.  `#if HAVE_IFUNC'.

Having to apply these guards on a per-function basis adds unnecessary
clutter to the file and makes its maintenance more error-prone.

We therefore reorganize the layout of the file in such a way that all
core implementations needing no `#ifdef's are placed first, followed
by all ifunc-dependent implementations, which can all be guarded by a
single `#if HAVE_IFUNC', greatly reducing the overall number of
required `#ifdef' macros.

libatomic/ChangeLog:

	* config/linux/aarch64/atomic_16.S: Reorganize functions in
	file.
	(HAVE_FEAT_LSE2): Delete.