llvm: Default to -mno-relax on RISC-V

Compiling on a RISC-V system fails with 'relocation R_RISCV_ALIGN
requires unimplemented linker relaxation; recompile with -mno-relax'.

Our default linker (ld.lld) doesn't support relaxation, so default to
no-relax so we don't generate object files the linker can't handle.

Reviewed by: mhorne
Sponsored by: Axiado
Differential Revision: https://reviews.freebsd.org/D25210