bsd-user: pull in target_arch_thread.h update target_arch_elf.h

Update target_arch_elf.h to remove thread_init. Move its contents to
target_arch_thread.h and rename to target_thread_init(). Update
elfload.c to call it. Create thread_os_thread.h to hold the os specific
parts of the thread and threat manipulation routines. Currently, it just
includes target_arch_thread.h. target_arch_thread.h contains the at the
moment unused target_thread_set_upcall which will be used in the future
when creating actual thread (i386 has this stubbed, but other
architectures in the bsd-user tree have real ones). FreeBSD doesn't do
AT_HWCAP, so remove that code. Linux does, and this code came from there.

These changes are all interrelated and could be brokend down, but seem
to represent a reviewable changeset since most of the change is boiler
plate.

Signed-off-by: Stacey Son <sson@FreeBSD.org>
Signed-off-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Kyle Evans <kevans@FreeBSD.org>