[RISCV] Support '.option arch' directive

The proposal of '.option arch' directive is https://github.com/riscv-non-isa/riscv-asm-manual/pull/67

Note: For '.option arch, +/-' directive, version number is not yet supported.

Reviewed By: luismarques, craig.topper

Differential Revision: https://reviews.llvm.org/D123515