uspace: use rtapi_get_time to implement rtapi_get_clocks if necessary

On non-x86 platforms there may be no tsc-like time source available
(e.g., arm).  The UBC arm port chose to use the "perf" interface,
but on the Samsung Arm Chromebook, I found that both gettimeofday
and clock_gettime are faster: Over 100k back-to-back invocations,
clock_gettime and gettimeofday both take about 850ns/invocation, while
the perf implementation takes 1500ns/invocation.
rtapi_bitops: fall back to implementation based on gcc intrinsics
rtapi_vsnprintf: Fix float formatting for !IS_IEEE754
rtapi_vsnprintf: arm IS_IEEE754

.. this gives the right results, and in about half the time as measured
by the test.