gcc: Do not set -march for arm64 for libatomic

libatomic has mind of its own when it comes to setting -march for arm64
which conflicts with -mcpu option we pass from environment in some cases
since we always pass -march/-mcpu in OE, its safe to remove this option

mcpu removal from cortex-a55 is no longer needed since the option
conflict is now removed from libatomic instead

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>