config/arm: use native machine build arguments

config/arm: use native machine build arguments

Letting the compiler decide is going to yield the best results for
native builds, so use native machine args usable for both GCC and Clang.

Signed-off-by: Juraj Linkeš <juraj.linkes@pantheon.tech>
Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
Acked-by: Jerin Jacob <jerinj@marvell.com>
Tested-by: Jerin Jacob <jerinj@marvell.com>
Tested-by: Pavan Nikhilesh <pbhagavatula@marvell.com>
Tested-by: Vimal Chungath <vcchunga@amazon.com>
Tested-by: Dharmik Thakkar <dharmik.thakkar@arm.com>