bsd-user: move arch specific defines out of elfload.c

Move the architecture specific defines to target_arch_elf.h and delete
them from elfload.c. Only retain ifdefs appropriate for i386 and x86_64.
Add the copyright/license comments, and guard ifdefs.

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