microlib support: Fix build with Arm Compiler 6 and MicroLib

Building binaries with MicroLib and Arm compiler 6 has been failing
because of a bug in Arm compiler 6. The compiler introduces a
symbol, `__scanf_mbtowc`, which is not present in MicroLib C library.

The present commit adds a weak reference to the symbol to allow a
successful linkage. This is a temporary fix until the bug in the
compiler is fixed. It should be removed after the compiler bug is fixed.

rtos/TARGET_CORTEX/TOOLCHAIN_ARM_MICRO/mbed_boot_arm_micro.c

@@ -68,3 +68,20 @@ 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
+#include <stdio.h>
+typedef int ScanfReadRec;
+MBED_WEAK long int _scanf_mbtowc(
+    int ignored,
+    FILE *p,
+    ScanfReadRec *sr,
+    int *charmap,
+    int exact
+)
+{
+    return 0;
+}