aarch64: add STO_AARCH64_VARIANT_PCS and DT_AARCH64_VARIANT_PCS

STO_AARCH64_VARIANT_PCS is a non-visibility st_other flag for marking
symbols that reference functions that may follow a variant PCS with
different register usage convention from the base PCS.

DT_AARCH64_VARIANT_PCS is a dynamic tag that marks ELF modules that
have R_*_JUMP_SLOT relocations for symbols marked with
STO_AARCH64_VARIANT_PCS (i.e. have variant PCS calls via a PLT).

	* elf/elf.h (STO_AARCH64_VARIANT_PCS): Define.
	(DT_AARCH64_VARIANT_PCS): Define.