When the linker doesn't have this feature, add -mno-relax to CFLAGS on RISC-V. Define the feature for ld.bfd, but not lld. If lld gains relaxation support in a newer version, we can enable it for those versions of lld in bsd.linker.mk. Reviewed by: mhorne Sponsored by: DARPA Differential Revision: https://reviews.freebsd.org/D22659