defined tokens for C, M, Ziscr extensions.
separate the base RV32 instructions from the RV64, for potential future
re-use in a RV32-only assembler, from which the RV64-tok can #include
scall, sbreak have been renamed (page 7 of spec),
necessitating some renaming in riscv64-asm.c
riscv-spec-20191213.pdf was used,
in which the "V" extension is not yet ratified.
available under https://riscv.org/technical/specifications/
Tables 16.5–16.7 do not list any "scall"
neither does the privileged spec
3 additional tokens not present in the tables were removed
note that this riscv64-asm.c still contains defects, which will
be addressed in another commit