Partially compensate for profiler overhead when reporting times

We now attempt to subtract the average profiler overhead when reporting
times; previously, times of outer loops included the profiling overhead
of the inner loops. The approach taken here is coarse, but provides
much better results than previously.
CodeGen_ARM now always replaces profiling_timer with halide_current_time_ns

Both Android and iOS disable user-mode access to the cycle counter in
stock builds; might as well assume that all ARM devices should avoid
using the cycle counter.