[RISCV] Add an option to emit the Tag_RISCV_arch attribute based on the assembler's subtarget

This adds an option to emit the command line -mattr/-march into the
attributes of an object file. This can be useful to get objdump to
disassemble instructions that aren't in the base without forcing
users to add a .attribute to the assembly file.

The binutils assembler does this by default.

Similar option exists for ARM. I will wire it to a clang option in
another patch. Similar to https://reviews.llvm.org/D31813

Reviewed By: asb, kito-cheng

Differential Revision: https://reviews.llvm.org/D148782