aarch64: Fix up __aarch64_cas16_acq_rel fallback

aarch64: Fix up __aarch64_cas16_acq_rel fallback

As mentioned in the PR, the fallback path when LSE is unavailable writes
incorrect registers to the memory if the previous content compares equal
to x0, x1 - it writes copy of x0, x1 from the start of function, but it
should write x2, x3.

2020-08-03  Jakub Jelinek  <jakub@redhat.com>

	PR target/96402
	* config/aarch64/lse.S (__aarch64_cas16_acq_rel): Use x2, x3 instead
	of x(tmp0), x(tmp1) in STXP arguments.

	* gcc.target/aarch64/pr96402.c: New test.

(cherry picked from commit 90b43856fdff7d96d93d22970eca8a86c56e0ddc)