usercopy: Move enum for arch_within_stack_frames()

This patch moves the arch_within_stack_frames() return value enum up in
the header files so that per-architecture implementations can reuse the
same return values.

Signed-off-by: Sahara <keun-o.park@darkmatter.ae>
Signed-off-by: James Morse <james.morse@arm.com>
[kees: adjusted naming and commit log]
Signed-off-by: Kees Cook <keescook@chromium.org>