boards: qemu_x86_64: enable icount for CONFIG_MP_NUM_CPUS=1

QEMU icount does not work correctly when more than 1 CPU is used,
but works fine with only 1 CPU enabled. So we can enable icount
on qemu_x86_64 when CONFIG_MP_NUM_CPUS=1. Sanitycheck goes from
~158 seconds to ~113 seconds on my dev machine.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>