aarch64: handle .variant_pcs directive in gas

In ELF objects the specified symbol is marked with STO_AARCH64_VARIANT_PCS.

gas/ChangeLog:

	* config/tc-aarch64.c (s_variant_pcs): New function.
	* doc/c-aarch64.texi: Document .variant_pcs.
	* testsuite/gas/aarch64/symbol-variant_pcs-1.d: New test.
	* testsuite/gas/aarch64/symbol-variant_pcs-1.s: New test.
	* testsuite/gas/aarch64/symbol-variant_pcs-2.d: New test.
	* testsuite/gas/aarch64/symbol-variant_pcs-2.s: New test.
7 files changed