[JITLink][RISCV] Support relaxable edges without relaxation pass

Relaxable edges are created unconditionally, even when the relaxation
pass will not run. However, they were not recognized by applyFixup
causing them to not be applied.

To support configurations without the relaxation pass, this patch adds
these relaxable edges to applyFixup:
- CallRelaxable: Can be treated as R_RISCV_CALL
- AlignRelaxable: Can simply be ignored

An alternative could be to unconditionally run the relaxation pass, even
in contexts where shouldAddDefaultTargetPasses returns false. However, I
could imagine there being use cases for disabling relaxation which
wouldn't be possible anymore then.

Reviewed By: StephenFan

Differential Revision: https://reviews.llvm.org/D153541