Add a stubbed-out version of the bsd-user fork's core dump support. This
allows elfload.c to be almost the same between what's upstream and
what's in qemu-project upstream w/o the burden of reviewing the core
dump support.

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