[RISCV] Support .variant_cc directive for the assembler.

The patch is split from D103435. The patch supported a new directive .variant_cc
that annotates function with STO_RISCV_VARIANT_CC. Symbols marked with
STO_RISCV_VARIANT_CC do not use standard calling conversion or use parameter not
passed in GPR/FPR.

Related: https://github.com/riscv/riscv-elf-psabi-doc/pull/190

Initial authored by: HsiangKai

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D138352