objtool: Track init section

For future usage of .init.text exclusion track the init section in the
instruction decoder and use the result in retpoline validation.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20220915111145.910334431@infradead.org