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.

Reported by: mhorne
Reviewed by: imp, mhorne
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D23076