MFC 355428,356504: Add a new "riscv-relaxations" linker feature.

355428:
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.

356504:
Add -mno-relax to CFLAGS in bsd.prog/lib.mk instead of bsd.cpu.mk.

bsd.cpu.mk is included by bsd.init.mk before bsd.linker.mk, so it
was always setting the flag since LINKER_FEATURES wasn't defined.