Rework the block device handle check to allow more robust device
classification. This is mostly usability issue - it can be quite confusing
for user when no disks are listed with lsdev.
Add more comments about what and why is done.
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D13026
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