objtool: Fix switch-table detection

Linus reported that GCC-7.3 generated a switch-table construct that
confused objtool. It turns out that, in particular due to KASAN, it is
possible to have unrelated .rodata usage in between the .rodata setup
for the switch-table and the following indirect jump.

The simple linear reverse search from the indirect jump would hit upon
the KASAN .rodata usage first and fail to find a switch_table,
resulting in a spurious 'sibling call with modified stack frame'
warning.

Fix this by creating a 'jump-stack' which we can 'unwind' during
reversal, thereby skipping over much of the in-between code.

This is not fool proof by any means, but is sufficient to make the
known cases work. Future work would be to construct more comprehensive
flow analysis code.

Reported-and-tested-by: Linus Torvalds <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Acked-by: Josh Poimboeuf <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

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