bsd-user/x86_64/target_arch_thread.h: Assume a FreeBSD target

bsd-user/x86_64/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/x86_64/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>