userspace: add z_is_in_user_syscall()

Certain types of system call validation may need to be pushed
deeper in the implementation and not performed in the verification
function. If such checks are only pertinent when the caller was
from user mode, we need an API to detect this situation.

This is implemented by having thread->syscall_frame be non-NULL
only while a user system call is in progress. The template for the
system call marshalling functions is changed to clear this value
on exit.

A test is added to prove that this works.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>