tests/benchmarks/timing: Unify the "standard zephyr timing" cases

It's worth using custom timing information on a few systems to save
cycles or gain precision.  But make the use of k_cycle_get_32() a
proper default instead of hardcoding all the platforms and failing to
build on new ones.  On Xtensa and RISC-V (and now x86_64) the cycle
informatoin from that call is a very fast wrapper around the native
counters anyway -- all you would save would be the function call
overhead.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>