Add the initcall tracer. It will be able to trace the initcalls which follow SMP initialization.
It is intended to be used with scripts/bootgraph.pl after some small improvements.
Note that it is not active after its init. To avoid tracing (and so crashing) before the whole tracing engine init,
you have to explicitly call start_initcall_trace() after do_pre_smp_initcalls() to enable it.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
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