modules: tf-m: Add Crosstool-NG toolchain support

Zephyr SDK is not supported on OS X, however, Zephyr toolchain can be
built on OS X using Crosstool-NG.

Currently, trusted-firmware-m CMakeLists.txt only supports `zephyr` and
`gnuarmemb` toolchains which prohibits building TF-M on OS X using
Zephyr toolchain built with Crosstool-NG.

docs:
https://docs.zephyrproject.org/latest/getting_started/toolchain_3rd_party_x_compilers.html#crosstool-ng



Signed-off-by: Devaraj Ranganna <devaraj.ranganna@linaro.org>