gcc-runtime: Avoid march conflicts with newer cortex-a55 CPUs

gcc-runtime: Avoid march conflicts with newer cortex-a55 CPUs

gcc-runtime/libatomic explicitly add -march=armv8-a+lse for all arch64
but cortex-a55 is armv8.2-a, which essentially conflicts, so let gcc
override it by not forcing the -mcpu option from TUNE_CCARGS

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