[RISCV] Allow llvm-objdump to disassemble objects with unrecognised versions of known extensions

This Moves ELFObjectFile to using
RISCVISAInfo::parseNormalizedArchString which does not ignore or produce
an error for ISA extensions with a version that isn't
recognised/supported by LLVM. As current GCC is marking its objects with
a higher version of the A, F, and D extensions than LLVM (see [extension
versioning
discussion](https://discourse.llvm.org/t/rfc-resolving-issues-related-to-extension-versioning-in-risc-v/68472)
this massively improves the usability of llvm-objdump with such
binaries.

This patch is functionally a cherry-pick of
af602edf and
91c6174c, though I've constructed the
backport manually because other changes unrelated to this specific fix
make a direct cherry pick difficult.