Fix ARMC6 linker error for the bare metal profile

The ARMC6 compiler inadvertently introduces the `_scanf_mbtowc` symbol
to the build. The commit provides a weak definition of the symbol to
satisfy the linker whenever the symbol is included. This affects ARM
Compiler 6 version 6.12 and earlier.

The compiler error was previously observed when the MICROLIB library is
used with the uARM toolchain. However, the weak definition was put in
`mbed-os/rtos` which is not included when the bare metal profile is used.

platform/source/mbed_retarget.cpp

@@ -1735,3 +1735,21 @@ MBED_WEAK const ticker_info_t *lp_ticker_get_info()
     };
     return &info;
 }
+
+
+// The below resolves the linker error generated by a bug in Arm Compiler 6
+// The compiler inadvertently introduces the
+// _scanf_mbtowc symbol to the build. The code below provides a weak reference
+// for the missing symbol.
+// Arm Compiler 6 version 6.12 and earlier versions are affected.
+typedef int ScanfReadRec;
+extern "C" MBED_WEAK long int _scanf_mbtowc(
+    int ignored,
+    FILE *p,
+    ScanfReadRec *sr,
+    int *charmap,
+    int exact
+)
+{
+    return 0;
+}

rtos/source/TARGET_CORTEX/TOOLCHAIN_ARM_MICRO/mbed_boot_arm_micro.c

@@ -68,21 +68,3 @@ MBED_WEAK void __aeabi_assert(const char *expr, const char *file, int line)
 {
     mbed_assert_internal(expr, file, line);
 }
-
-// The below resolves the linker error generated by a bug in Arm Compiler 6
-// When building with MicroLib, the compiler inadvertently introduces the
-// _scanf_mbtowc symbol to the build. The code below provides a weak reference
-// for the missing symbol.
-// Arm Compiler 6 version 6.12 and earlier versions are affected.
-#include <stdio.h>
-typedef int ScanfReadRec;
-MBED_WEAK long int _scanf_mbtowc(
-    int ignored,
-    FILE *p,
-    ScanfReadRec *sr,
-    int *charmap,
-    int exact
-)
-{
-    return 0;
-}