[llvm-objdump/llvm-readobj/obj2yaml/yaml2obj] Support STO_RISCV_VARIANT_CC and DT_RISCV_VARIANT_CC

STO_RISCV_VARIANT_CC marks that a symbol uses a non-standard calling
convention or the vector calling convention.

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

Differential Revision: https://reviews.llvm.org/D107949