arch/x86: refactor z_do_read_cpu_timestamp32()

This function, like its cousin z_tsc_read(), can be shared between
subarches. Note: it's unclear to me if we actually need these to be
two separate functions (do we need serialization?) or if they can
be combined into one function. Add that to the to-do list.

Signed-off-by: Charles E. Youse <charles.youse@intel.com>