Merge pull request #10704 from hugueskamba/fix-microlib-with-arm-compiler-6

microlib support: Fix build with Arm Compiler 6 and MicroLib

Author: 0xc0170

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;
+}