Add a new "riscv-relaxations" linker feature.

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