arch: arm: include GCC -mcmse compile option for secure firmware

Make GCC compile with the -mcmse compile option, if we are
building a Secure firmware. The option will make Security
Extensions for secure executables available, and will set
the corresponding compile-time indicator flag, accordingly:
(i.e. __ARM_FEATURE_CMSE=3).

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>

arch/arm/core/cortex_m/tz/CMakeLists.txt

@@ -1 +1,5 @@
-zephyr_sources_ifdef(CONFIG_ARM_SECURE_FIRMWARE arm_core_tz.c)
+# '-mcmse' enables the generation of code for the Secure state of the ARMv8-M
+# Security Extensions. This option is required when building a Secure firmware.
+zephyr_compile_options(-mcmse)
+
+zephyr_sources(arm_core_tz.c)