New machine arches for arm: earmv7eb and earmv7hfeb

[Matt Thomas <matt%3am-software.com@localhost>]

No one has tried run NetBSD on a ARMv7A CPU in big endian since NetBSD didn't
have proper support for them.  Previous big endian CPUs used a format [BE-32]
which all instructions and data where in big-endian.  Starting with ARMv7A
(e.g. Cortex processors) only data is in big-endian, instructions are in
little-endian [BE-8].

Since these are incompatible, I've added two new MACHINE_ARCH for arm to
reflect this: earmv7eb and earmv7hfeb which default to cortex-a8 cpu, BE8
mode, and soft and hard float respectively.

Only two machine aliases use them: evbearmv7-eb and evbearmv7hf-eb.