Due to a known issue in Mbed TLS's architecture determination
(ARMmbed/mbedtls#1077), we get the error

    error: inline assembly requires more registers than available

when compiling `bignum.c` for Cortex-M0/0+/1/M23 which do not have
the macro `__thumb2__` set by the compiler.

The workaround is to define the macro `MULADDC_CANNOT_USE_R7` which
is already defined by Mbed CLI 1 but missing in our CMake support.

Fixes ARMmbed/mbed-os-example-lorawan#220