[AArch64] Emit tighter strong atomic compare-exchange loop when comparing against...

[AArch64] Emit tighter strong atomic compare-exchange loop when comparing against zero

	* config/aarch64/aarch64.c (aarch64_split_compare_and_swap):
	Emit CBNZ inside loop when doing a strong exchange and comparing
	against zero.  Generate the CC flags after the loop.

	* gcc.target/aarch64/atomic_cmp_exchange_zero_strong_1.c: New test.

(backported c5dcf8e544c5e98f1160ec390111b28d11d73b68)

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@248832 138bc75d-0d04-0410-961f-82ee72b054a4

Change-Id: I5204c56a8d3c11b0fde6697007d601cbafdb3ccf