lib: processor: hosted: fix compilation warning in metal_cpu_yield

Fix cross inclusion between metal/sys.h and /processor/hosted/cpu.h
replacing metal_cpu_yield() by the zephyr k_yield().

Issue reported by Zephyr twister CI test:

libmetal/lib/include/metal/processor/hosted/cpu.h:19:9:
error: implicit declaration of function ‘metal_wait_usec’
   19 |         metal_wait_usec(10);
      |         ^~~~~~~~~~~~~~~
libmetal/lib/include/metal/system/zephyr/sys.h:46:20:
error: conflicting types for ‘metal_wait_usec’; have ‘void(uint32_t)’
   46 | static inline void metal_wait_usec(uint32_t usec_to_wait)
      |                    ^~~~~~~~~~~~~~~
libmetal/lib/include/metal/processor/hosted/cpu.h:19:9:
note: previous implicit declaration of ‘metal_wait_usec’ with
type ‘void(uint32_t)’ {aka ‘void(unsigned int)’}
   19 |         metal_wait_usec(10);
      |         ^~~~~~~~~~~~~~~

Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@foss.st.com>

lib/system/zephyr/sys.h

@@ -16,15 +16,14 @@
 #ifndef __METAL_ZEPHYR_SYS__H__
 #define __METAL_ZEPHYR_SYS__H__
 
-#include <metal/cpu.h>
 #include <stdlib.h>
 #include <zephyr/kernel.h>
 
 #ifdef __cplusplus
 extern "C" {
 #endif
 
-#define metal_yield() metal_cpu_yield()
+#define metal_yield() k_yield()
 
 #define METAL_INIT_DEFAULTS				\
 {							\