Fix for unit test build on Windows machines
Fix for unit test build on Windows machines

In case MINGW is detected - define MBED_WEAK to be empty, as Windows
executables cannot handle the weak attribute.

andcor02/mbed-os,andcor02/mbed-os,mbedmicro/mbed,andcor02/mbed-os,kjbracey-arm/mbed,andcor02/mbed-os,mbedmicro/mbed,kjbracey-arm/mbed,andcor02/mbed-os,kjbracey-arm/mbed,mbedmicro/mbed,kjbracey-arm/mbed,mbedmicro/mbed,andcor02/mbed-os,mbedmicro/mbed