[TF-M] Build issues with GNUARM with 7946bdd3

The merge of "Build: Add support for specifying GNUARM_PREFIX" appears to break GNUARM builds.

[100%] Linking C static library mbedcrypto.a /usr/bin/sh: CMAKE_GNUARM_AR-NOTFOUND: command not found make[5]: *** [library/mbedcrypto.a] Fel 127 make[4]: *** [library/CMakeFiles/mbedcrypto.dir/all] Fel 2 make[3]: *** [all] Fel 2 make[2]: *** [secure_fw/partitions/crypto/mbedcrypto_lib-prefix/src/mbedcrypto_lib-stamp/mbedcrypto_lib-build] Fel 2 make[1]: *** [secure_fw/partitions/crypto/CMakeFiles/mbedcrypto_lib.dir/all] Fel 2 make: *** [all] Fel 2

Excluding this commit is a workaround.

This is on Win 10.

Cheers, Thomas