FreeBSD's get_mcontext doesn't return any vfp data. Instead, it zeros
out the vfp feilds (and all the spare fields). Impelement this
behavior. We're still missing the sysarch(ARM_GET_VFPCONTEXT) syscall,
though.

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