elf_common.h: Add STO_AARCH64_VARIANT_PCS

This is used to mark symbols that use a variant procedure call with
a different calling convention to the main ABI.

Reviewed by: kib, imp
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D44868

(cherry picked from commit d7ac42676dc403cd31d4a6c9a91a2ed5680cca86)