--discard-locals (-X) instructs the linker to remove local .L* symbols,
which occur a lot due to label differences for linker relaxation. The
arm port has a similar need and passes -X to ld.
In contrast, the RISC-V port does not pass -X to ld and rely on the
default --discard-locals in GNU ld's riscv port. The arm way is more
conventional (compiler driver instead of the linker customizes the
default linker behavior) and works with lld.