__cxa_pure_virtual changes

- rename stdio.cpp to retarget.cpp, since it doesn't contain only stdio-related
  code
- move __cxa_pure_virtual from exit.c to retarget.cpp where it belongs.

libraries/mbed/common/exit.c

@@ -15,7 +15,6 @@
  */
 #include "semihost_api.h"
 #include "mbed_interface.h"
-#include "toolchain.h"
 
 #ifdef TOOLCHAIN_GCC_CW
 // TODO: Ideally, we would like to define directly "_ExitProcess"
@@ -35,9 +34,3 @@ void exit(int return_code) {
 
     while (1);
 }
-
-WEAK void __cxa_pure_virtual(void);
-WEAK void __cxa_pure_virtual(void) {
-    exit(1);
-}
-

libraries/mbed/common/retarget.cpp

@@ -18,6 +18,7 @@
 #include "FileSystemLike.h"
 #include "FilePath.h"
 #include "serial_api.h"
+#include "toolchain.h"
 #include <errno.h>
 
 #if defined(__ARMCC_VERSION)
@@ -386,4 +387,9 @@ namespace __gnu_cxx {
         error("Exception");
     }
 }
+extern "C" WEAK void __cxa_pure_virtual(void);
+extern "C" WEAK void __cxa_pure_virtual(void) {
+    exit(1);
+}
+
 #endif