bsd-user: introduce target.h

bsd-user: introduce target.h

Create target.h. This file is intended to be simple and describe basic
things about the architecture. If something is a basic feature of the
architecture, it belongs here. Should we need something that's per-BSD
there will be a target-os.h that will live in the per-bsd directories.

Define regpairs_aligned to reflect whether or not registers are 'paired'
for 64-bit arguments or not. This will be false for all 64-bit targets,
and will be true on those architectures that pair (currently just armv7
and powerpc on FreeBSD 14.x).

Signed-off-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Kyle Evans <kevans@FreeBSD.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>