[llvm-readobj/elf] - AArch64: Handle AARCH64_VARIANT_PCS for GNUStyle

It mimics the GNU readelf where it prints a [VARIANT_PCS] for symbols
with st_other with STO_AARCH64_VARIANT_PCS.

Reviewed By: grimar, MaskRay

Differential Revision: https://reviews.llvm.org/D93044