kernel: Add alternative _arch_switch context switch primitive

The existing __swap() mechanism is too high level for some
applications because of its scheduler-awareness.  This introduces a
new _arch_switch() mechanism, which is a simpler primitive that looks
like:

    void _arch_switch(void *handle, void **old_handle_out);

The new thread handle (typically just a stack pointer) is specified
explicitly instead of being picked up from the scheduler by
per-architecture code, and on return the "old" thread handle that got
switched out is returned through the pointer.

The new primitive (currently available only on xtensa) is selected
when CONFIG_USE_SWITCH is "y".  A new C _Swap() implementation based
on this primitive is then added which operates compatibly.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>

include/kernel.h

@@ -469,6 +469,18 @@ struct k_thread {
 	k_thread_stack_t *stack_obj;
 #endif /* CONFIG_USERSPACE */
 
+#if defined(CONFIG_USE_SWITCH)
+	/* When using __switch() a few previously arch-specific items
+	 * become part of the core OS
+	 */
+
+	/* _Swap() return value */
+	int swap_retval;
+
+	/* Context handle returned via _arch_switch() */
+	void *switch_handle;
+#endif
+
 	/* arch-specifics: must always be at the end */
 	struct _thread_arch arch;
 };