Libatomic: Make ifunc selector behavior contingent on importing file

By querying previously-defined file-identifier macros, `host-config.h'
is able to get information about its environment and, based on this
information, select more appropriate function-specific ifunc
selectors.  This reduces the number of unnecessary feature tests that
need to be carried out in order to find the best atomic implementation
for a function at run-time.

An immediate benefit of this is that we can further fine-tune the
architectural requirements for each atomic function without risk of
incurring the maintenance and runtime-performance penalties of having
to maintain an ifunc selector with a huge number of alternatives, most
of which are irrelevant for any particular function.  Consequently,
for AArch64 targets, we relax the architectural requirements of
`compare_exchange_16', which now requires only LSE as opposed to the
newer LSE2.

The new flexibility provided by this approach also means that certain
functions can now be called directly, doing away with ifunc selectors
altogether when only a single implementation is available for it on a
given target.  As per the macro expansion framework laid out in
`libatomic_i.h', such functions should have their names prefixed with
`__atomic_' as opposed to `libat_'.  This is the same prefix applied
to function names when Libatomic is configured with
`--disable-gnu-indirect-function'.

To achieve this, these functions unconditionally apply the aliasing
rule that at present is conditionally applied only when libatomic is
built without ifunc support, which ensures that the default
`libat_##NAME' is accessible via the equivalent `__atomic_##NAME' too.
This is ensured by using the new `ENTRY_ALIASED' macro.

Finally, this means we are able to do away with a whole set of
function aliases that were needed until now, thus considerably
cleaning up the implementation.

libatomic/ChangeLog:

	* config/linux/aarch64/atomic_16.S: Remove unnecessary
	aliasing.
	(LSE): New.
	(ENTRY_ALIASED): Likewise.
	* config/linux/aarch64/host-config.h (LSE_ATOP): New.
	(LSE2_ATOP): Likewise.
	(LSE128_ATOP): Likewise.
	(IFUNC_COND_1): Make its definition conditional on above 3
	macros.
	(IFUNC_NCOND): Likewise.