libatomic: Add rcpc3 128-bit atomic operations for AArch64

The introduction of the optional RCPC3 architectural extension for
Armv8.2-A upwards provides additional support for the release
consistency model, introducing the Load-Acquire RCpc Pair Ordered, and
Store-Release Pair Ordered operations in the form of LDIAPP and STILP.

These operations are single-copy atomic on cores which also implement
LSE2 and, as such, support for these operations is added to Libatomic
and employed accordingly when the LSE2 and RCPC3 features are detected
in a given core at runtime.

libatomic/ChangeLog:

	* config/linux/aarch64/atomic_16.S (libat_load_16): Add LRCPC3
	variant.
	(libat_store_16): Likewise.
	* config/linux/aarch64/host-config.h (HWCAP2_LRCPC3): New.
	(LSE2_LRCPC3_ATOP): Previously LSE2_ATOP.  New ifuncs guarded
	under it.
	(has_rcpc3): New.