x86: Allow x86-32 instruction decoder selftest on x86-64

Pass $(CONFIG_64BIT) to the x86 insn decoder selftest in case we are
decoding 32bit code on x86-64, which will happen when building kernel
with ARCH=i386 on x86-64.

Signed-off-by: Masami Hiramatsu <mhiramat@redhat.com>
Cc: Jim Keniston <jkenisto@us.ibm.com>
Cc: Ingo Molnar <mingo@elte.hu>
LKML-Reference: <20090828221319.8778.88508.stgit@localhost.localdomain>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>