Assembler command-line option to allow 'C' (compressed) instructions but not to mutate input assembly

[asb]

It seems desirable to have a command-line option where c.addi sp, -16 is accepted but addi sp, sp, -16 isn't automatically encoded as a compressed instruction.

[cmuellner]

Using asm directives should be a better solution than a command-line flag, as they allow more fine-grained control.

The use case of preventing compressed instructions was addressed by the norvc asm directive.
The proposed exact directive (see riscv-non-isa/riscv-asm-manual#122) follows the idea outlined in this issue to compress only if instructions have the c. prefix.

Closing as partially done and partially tracked elsewhere.

[lenary]

Heh, I didn't even know about this, or didn't recall it. Nice to fix a request from 2017 :)