[RISCV] Support .option {no}exact (#122483)
This implements [the `.option exact` and `.option noexact`
proposal](https://github.com/riscv-non-isa/riscv-asm-manual/pull/122)
for RISC-V.
`.option exact` turns off:
- Compression
- Branch Relaxation
- Linker Relaxation
`.option noexact` turns these back on, and is also the default, matching
the current behaviour.
