[RISCV] Relax march string order constraint (#78120)

Follow
https://github.com/riscv-non-isa/riscv-toolchain-conventions/pull/14 by
dropping the order requirement of `-march`.

1. single-letter extension can be arbitrary order
    - march=rv32iamdf 
2. single-letter extension and multi-letter extension can be mixed
    - march=rv32i_zihintntl_m_a_f_d_svinval
3. multi-letter extension need seperate the following extension by
underscore, otherwise it will be intreprete as one extension.
    - march=rv32i_zbam -> i,zbam
    - march=rv32i_zba_m -> i,zba,m