microlib support: Specify compiler version unable to build without the fix

Author: hugueskamba

rtos/TARGET_CORTEX/TOOLCHAIN_ARM_MICRO/mbed_boot_arm_micro.c

@@ -72,7 +72,8 @@ MBED_WEAK void __aeabi_assert(const char *expr, const char *file, int 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
+// 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(