Move some more common stuff up to Makefile.inc. In particular, the no
simd / no float stuff is centeralized here. Also centralise
-ffreestanding since it is specified everywhere.

This, along with a change to share/mk/bsd.cpu.mk to include -mno-avx2
in CFLAGS_NO_SIMD should fix building for newer machines (eg with
CPUTYPE=haswell) where clang was generating avx2 instructions.

Sponsored by: Netflix