bsd-user: Remove all non-x86 code from elfload.c

bsd-user only builds x86 at the moment. Remove all non x86 code from
elfload.c. We'll move the x86 code to {i386,x86_64}/target_arch_elf.h
and bring it that support code from the forked bsd-user when the time
comes.

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