bsd-user/arm/target_arch_thread.h: Assume a FreeBSD target

bsd-user/arm/target_arch_thread.h: Assume a FreeBSD target

Since we can't run on anything else, assume for the moment that this is
a FreeBSD target. In the future, we'll need to handle this properly
via some include file in bsd-user/*bsd/arm/mumble.h. There's a number
of other diffs that would be needed to make things work on OtherBSD,
so it doesn't make sense to preseve this one detail today.

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