Commit 396e0b8e ("x86/orc: Make it callthunk aware") attempted to
deal with the fact that function prefix code didn't have ORC coverage.
However, it didn't work as advertised.  Use of the "null" ORC entry just
caused affected unwinds to end early.

The root cause has now been fixed with commit 5743654f ("objtool:
Generate ORC data for __pfx code").

Revert most of commit 396e0b8e ("x86/orc: Make it callthunk aware").
The is_callthunk() function remains as it's now used by other code.

Link: https://lore.kernel.org/r/a05b916ef941da872cbece1ab3593eceabd05a79.1684245404.git.jpoimboe@kernel.orgSigned-off-by: Josh Poimboeuf <jpoimboe@kernel.org>