Remove lex file duplication

[SamuelmsWong-zz]

lex-hitachi-sh.c and lex-riscv.c differ only in the tokens for their assembly instructions, so the rest (tokens for Sunflower commands and the lexical analyser function) is all duplicated.

[phillipstanleymarbell]

One approach would be to split off the common %token T_* at the start of the .ys into a sf-common-tokens.y and to similarly split off the common grammar rules into a sf-common-grammar.y. @samuelmswong @rjlv2 @jan-heck : if any of you are interested...

[SamuelmsWong-zz]

Hi. I can probably get started on this in a few weeks.

[phillipstanleymarbell]

That's great!