[AArch64] Implement .variant_pcs directive

A dynamic linker with lazy binding support may need to handle variant
PCS function symbols specially, so an ELF symbol table marking
STO_AARCH64_VARIANT_PCS [1] was added to address this.

Function symbols that follow the vector PCS are marked via the
.variant_pcs assembler directive, which takes a single parameter
specifying the symbol name and sets the STO_AARCH64_VARIANT_PCS st_other
flag in the object file.

[1] https://github.com/ARM-software/abi-aa/blob/master/aaelf64/aaelf64.rst#st-other-values

Reviewed By: sdesmalen

Differential Revision: https://reviews.llvm.org/D89138