Rename core/combo/arch/x86/x86-atom.mk to core/combo/arch/x86/atom.mk.

Otherwise wrong set of memory/string functions is chosen when building atom
image (in Bionic we have libc/arch-x86/atom/atom.mk). Plus this naming is
consistent with other x86 architectures.

Change-Id: I21e899534e7ce10530474a22ceba770422b39d8d
Signed-off-by: Alexander Ivchenko <alexander.ivchenko@intel.com>