tests: kernel/smp_boot_delay: make test more generic

This makes the test a bit more generic so it can run on
other SMP enabled platforms. Though, we don't need CI
running this on every SMP platforms as this is not
testing the mechanism of bringing up a CPU. That is
being tested on the other SMP test. Therefore, only
enable qemu_x86_64 to be testing in CI so that this
feature will still be tested on CI.

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