[RISCV] Unify the arch string parsing logic to to RISCVISAInfo.

How many place you need to modify when implementing a new extension for RISC-V?

At least 7 places as I know:

- Add new SubtargetFeature at RISCV.td
- -march parser in RISCV.cpp
- RISCVTargetInfo::initFeatureMap@RISCV.cpp for handling feature vector.
- RISCVTargetInfo::getTargetDefines@RISCV.cpp for pre-define marco.
- Arch string parser for ELF attribute in RISCVAsmParser.cpp
- ELF attribute emittion in RISCVAsmParser.cpp, and make sure it's in
  canonical order...
- ELF attribute emittion in RISCVTargetStreamer.cpp, and again, must in
  canonical order...

And now, this patch provide an unified infrastructure for handling (almost)
everything of RISC-V arch string.

After this patch, you only need to update 2 places for implement an extension
for RISC-V:
- Add new SubtargetFeature at RISCV.td, hmmm, it's hard to avoid.
- Add new entry to RISCVSupportedExtension@RISCVISAInfo.cpp or
  SupportedExperimentalExtensions@RISCVISAInfo.cpp .

Most codes are come from existing -march parser, but with few new feature/bug
fixes:
- Accept version for -march, e.g. -march=rv32i2p0.
- Reject version info with `p` but without minor version number like `rv32i2p`.

Differential Revision: https://reviews.llvm.org/D105168

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