arch: arm: Remove -march compiler flag
arch: arm: Remove -march compiler flag

The ARM GCC `-march` compiler flag is completely redundant when the
`-mcpu` flag is specified, since the `-mcpu` selects the target ARM
architecture as well as CPU-specific optimisations.

In fact, it is disadvantageous to specify both `-march` and `-mcpu`
flags because the `-march` flag overrides and disables any CPU-specific
optimisations enabled by the `-mcpu` flag.

Signed-off-by: Stephanos Ioannidis <dc76e9f0c0006e8f919e0c515c66dbba3982f785@stephanos.io>

zephyrproject-rtos/zephyr,Vudentz/zephyr,galak/zephyr,nashif/zephyr,finikorg/zephyr,nashif/zephyr,finikorg/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,nashif/zephyr,Vudentz/zephyr,Vudentz/zephyr,finikorg/zephyr,galak/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,Vudentz/zephyr,Vudentz/zephyr