rtems: Deprecate use of _RTEMS_version

Close #3970.

cpukit/include/rtems/system.h

@@ -35,12 +35,11 @@ extern "C" {
 
 #ifndef ASM
 
-/**
- *  The following is the extern for the RTEMS version string.
- *
- *  @note The contents of this string are CPU specific.
+/*
+ * The use of this variable is deprecated, use rtems_get_version_string()
+ * instead.
  */
-extern const char _RTEMS_version[];
+extern RTEMS_DEPRECATED const char _RTEMS_version[];
 
 /**
  *  The following is the extern for the RTEMS copyright string.

cpukit/sapi/src/getversionstring.c

@@ -19,6 +19,8 @@
 #include "config.h"
 #endif
 
+#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
+
 #include <rtems.h>
 
 const char *rtems_get_version_string(void)

testsuites/sptests/spsize/size.c

@@ -18,6 +18,8 @@
 #include "config.h"
 #endif
 
+#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
+
 #include <rtems/config.h>
 #include <rtems/score/copyrt.h>
 #include <rtems/rtems/clock.h>