Specifying custom extensions in the -march string

[asb]

The string accepted by -march currently deviates from the ISA naming strings described in chapter 22 of the RISC-V User ISA manual. Should compilers accept strings in the form march=rv32imcXfoobar_Xbazqux, or a more usual -march=rv32imc+foobar+bazqux, or perhaps even -march=rv32imc -mext=+foobar+bazqux (thus separating the extension namespace from other target-specific options that might be specified in -march).

[kito-cheng]

How about -march=rv32imcXfoobarXbazqux? underscore seems ugly...

For our internal porting we using -march=rv32imcXfoobar but we don't have more than one extension yet.