ipipe: add latency tracer
The latency tracer is a variant of ftrace's 'function' tracer providing detailed information about the current interrupt state at each function entry (i.e. virtual interrupt flag and CPU interrupt disable bit). This commit introduces the generic tracer code, which builds upon the regular ftrace API. The arch-specific code should provide for ipipe_read_tsc(), a helper routine returning a 64bit monotonic time value for timestamping purpose. HAVE_IPIPE_TRACER_SUPPORT should be selected by the arch-specific code for enabling the tracer, which in turn makes CONFIG_IPIPE_TRACE available from the Kconfig interface.