arch: arm: cortex_m: Add tz_ns.h

Provide a TZ_SAFE_ENTRY_FUNC() macro for wrapping non-secure entry
functions in calls to k_sched_lock()/k_sched_unlock()

Provide a __TZ_WRAP_FUNC() macro which helps in creating a function
that "wraps" another in a preface and postface function call.

	int foo(char *arg); // Implemented somewhere else.
	int __attribute__((naked)) foo_wrapped(char *arg)
	{
		WRAP_FUNC(bar, foo, baz);
	}

is equivalent to

	int foo(char *arg); // Implemented somewhere else.
	int foo_wrapped(char *arg)
	{
		bar();
		int res = foo(arg);
		baz();
		return res;
	}

This commit also adds tests for __TZ_WRAP_FUNC().

Signed-off-by: Øyvind Rønningstad <oyvind.ronningstad@nordicsemi.no>