bsd-user/freebsd: Create common target_os_ucontext.h file

bsd-user/freebsd: Create common target_os_ucontext.h file

FreeBSD has a MI ucontext structure that contains the MD mcontext
machine state and other things that are machine independent. Create an
include file for all the ucontext stuff. It needs to be included in the
arch specific files after target_mcontext is defined. This is largely
copied from sys/_ucontext.h with the comments about layout removed
because we don't support ancient FreeBSD binaries.

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