Update tools/toolchains/mbed_toolchain.py
Update tools/toolchains/mbed_toolchain.py

Co-Authored-By: Graham Hammond <aaae4df1ac2b12e272df8acead61285a3ebbd25e@arm.com>
andcor02/mbed-os,mbedmicro/mbed,andcor02/mbed-os,andcor02/mbed-os,mbedmicro/mbed,mbedmicro/mbed,kjbracey-arm/mbed,andcor02/mbed-os,kjbracey-arm/mbed,mbedmicro/mbed,andcor02/mbed-os,kjbracey-arm/mbed,mbedmicro/mbed,andcor02/mbed-os,kjbracey-arm/mbed