AArch64: Fix PR103085

AArch64: Fix PR103085

The stack protector implementation hides symbols in a const unspec, which means
movdi/movsi patterns must always support const on symbol operands and
explicitly strip away the unspec. Do this for the recently added GOT
alternatives. Add a test to ensure stack-protector tests GOT accesses as well.

2021-11-05  Wilco Dijkstra  <wdijkstr@arm.com>

	PR target/103085
	* config/aarch64/aarch64.c (aarch64_mov_operand_p): Strip the salt
	first.
	* config/aarch64/constraints.md: Support const in Usw.

gcc/testsuite/
	PR target/103085
	* gcc.target/aarch64/pr103085.c: New test