toolchain: xcc: define __deprecated only if not already defined

Some tests explicitly test for deprecated functions and they
override the macro __deprecated to avoid compiler warnings.
So, for xcc/xt-clang, only define __deprecated if it has not
already been defined.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>

Author: dcpleung

include/zephyr/toolchain/xcc.h

@@ -15,7 +15,9 @@
  * XCC does not support using deprecated attribute in enum,
  * so just nullify it here to avoid compilation errors.
  */
+#ifndef __deprecated
 #define __deprecated
+#endif
 
 #define __in_section_unique(seg) \
 	__attribute__((section("." STRINGIFY(seg) "." STRINGIFY(__COUNTER__))))