objtool: Add IBT/ENDBR decoding

Intel IBT requires the target of any indirect CALL or JMP instruction
to be the ENDBR instruction; optionally it allows those two
instructions to have a NOTRACK prefix in order to avoid this
requirement.

The kernel will not enable the use of NOTRACK, as such any occurence
of it in compiler generated code should be flagged.

Teach objtool to Decode ENDBR instructions and WARN about NOTRACK
prefixes.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lore.kernel.org/r/20220308154319.645963517@infradead.org