remove dups in x86 and x86_64 variables

remove dups in x86 and x86_64 variables

Currently the Xen build uses x86 and x86_64 variables as well as
CONFIG_X86 and CONFIG_X86_64. This just removes the duplication. The
CONFIG_ variables are now managed by Kconfig but existed previously so
this duplication existed prior to the Kconfig migration.

Signed-off-by: Doug Goldstein <cardoe@cardoe.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Feng Wu <feng.wu@intel.com>
$(CONFIG_X86_64) -> y in x86 makefiles.
$(CONFIG_X86_64) -> $(CONFIG_X86) in non-x86 makefiles.

Signed-off-by: Jan Beulich <jbeulich@suse.com>