Tidy up bsd.cpu.mk for X86 CPUs:

Tidy up bsd.cpu.mk for X86 CPUs:

- Do not limit recent processors to "prescott" class for i386 target.  There
is no reason for this hack because clang is default now.  On top of that, it
will only grow indefinitely over time.
- Add more CPUTYPEs, i.e., "athlon-fx", "core-avx2", "atom", "penryn", and
"yonah".  Note "penryn" and "yonah" are intentionally undocumented because
they are not supported by gcc and marked deprecated by clang.
- Add more CPUTYPE aliases, i.e., "barcelona" (-> amdfam10), "westmere" and
"nehalem" (-> corei7).  Note these are intentionally undocumented because
they are not supported by (base) gcc and/or clang.  However, LLVM (backend)
seems to "know" the differences.  Most likely, they were deprecated with
other vendor code names and clang did not bother implementing them at all.
- Add i686 to MACHINE_CPU for "c3-2" (VIA Nehemiah).  Both gcc & clang treat
it like an i686-class processor.
- Add IDT "winchip2" and "winchip-c6" for completeness (undocumented).
- Order processors per make.conf example, i.e., CPU vendors and models.
- Tidy up make.conf example, i.e., remove "by gcc" (because we have aliases)
and remove "prescott" from AMD64 architecture (because it is not correct).