x86: wrap MCA_bus test around an ifdef

Only test for MCA_bus if support for MCA is compiled in.
Also, for x86_64, write the code inside the conditional
for consistency with i386. It won't bite us, since it'll
probably never select CONFIG_MCA anyway.

Signed-off-by: Glauber Costa <gcosta@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>