AArch64: Fix __sync_val_compare_and_swap [PR111404]

__sync_val_compare_and_swap may be used on 128-bit types and either calls the
outline atomic code or uses an inline loop.  On AArch64 LDXP is only atomic if
the value is stored successfully using STXP, but the current implementations
do not perform the store if the comparison fails.  In this case the value
returned is not read atomically.

gcc/ChangeLog:
	PR target/111404
	* config/aarch64/aarch64.cc (aarch64_split_compare_and_swap):
	For 128-bit store the loaded value and loop if needed.

libgcc/ChangeLog:
	PR target/111404
	* config/aarch64/lse.S (__aarch64_cas16_acq_rel): Execute STLXP using
	either new value or loaded value.