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.