libatomic: Cleanup AArch64 ifunc selection

Simplify and cleanup ifunc selection logic.  Since LRCPC3 does
not imply LSE2, has_rcpc3() should also check LSE2 is enabled.

Passes regress and bootstrap, OK for commit?

libatomic:
	* config/linux/aarch64/host-config.h (has_lse2): Cleanup.
	(has_lse128): Likewise.
	(has_rcpc3): Add early check for LSE2.