Xtensa port: Removed warnings related to function _NanoFatalErrorHandler.

This function needs to be decalred in a file included by _thread_entry.
It also needs to have exit function declared as not returning.

Change-Id: I2a01e7408cf70266351ae5089f45b5d9d009fabe
Signed-off-by: Mazen NEIFER <mazen@nestwave.com>

arch/xtensa/core/fatal.c

@@ -16,7 +16,7 @@ const NANO_ESF _default_esf = {
 	0xdeaddead, /* pc */
 };
 
-extern void exit(int exit_code);
+FUNC_NORETURN void exit(int exit_code);
 
 /**
  *
@@ -36,7 +36,8 @@ extern void exit(int exit_code);
  *
  * @return This function does not return.
  */
-void _NanoFatalErrorHandler(unsigned int reason, const NANO_ESF *pEsf)
+FUNC_NORETURN void _NanoFatalErrorHandler(unsigned int reason,
+					  const NANO_ESF *pEsf)
 {
 	switch (reason) {
 	case _NANO_ERR_INVALID_TASK_EXIT:

arch/xtensa/include/kernel_arch_func.h

@@ -78,6 +78,9 @@ static inline void _IntLibInit(void)
 {
 }
 
+FUNC_NORETURN void _NanoFatalErrorHandler(unsigned int reason,
+					  const NANO_ESF *pEsf);
+
 #include <stddef.h> /* For size_t */
 
 #ifdef __cplusplus