[PATCH 0/5] RISC-V: Relax the -march string for accept any order

[Kito Cheng <kito.cheng@sifive.com>]

Do you know how to build a ISA string with following extension?
- g
- c
- zba
- zbs
- svnapot
- zve64d
- zvl128b

Don't trial and error with your gcc and don't read RISC-V ISA spec! OK, I believe it's impossible for most people, even I work for RISC-V so many years, I remember most of the rule of the the canonical order, it's still hard to order that right in short time...

So I think it's time to relax that for the -march string inputs, since we have so many extension today, but we still keep the canonicalization within the compiler, because we need that to handle multi-lib and also it's easier to compare different ISA string.

This patch break into serveral part:
1) Small refactor patch
2) Change the way of parsing ISA string.
3) Remove unused functions
4) Update test cases
5) Update document