arch/arc: always needs -matomic with atomic extensions

arch/arc: always needs -matomic with atomic extensions

As reported by Alexey in:
    https://patchwork.ozlabs.org/patch/1087480/
    https://patchwork.ozlabs.org/patch/1087471/

when BR2_ARC_ATOMIC_EXT is enabled, -matomic needs to always be passed
to the compiler to allow atomic instructions to be used. So instead of
passing them through the command-line CFLAGS, we enforce them in the
toolchain wrapper directly.

Reported-by: Alexey Brodkin <Alexey.Brodkin@synopsys.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Alexey Brodkin <Alexey.Brodkin@synopsys.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Acked-by: Alexey Brodkin <abrodkin@synopsys.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>