Inside LLVM, Functions become marked with exposesReturnsTwice() if they

Inside LLVM, Functions become marked with exposesReturnsTwice() if they
call a setjmp-type function (protyped with __attribute__((returns_twice)).

LLVM anticipates the longjmp type function will perform a direct branch
back (rather of a push;ret combo, almost certainly due to CET
shadow-stack coherency difficulties).  Since we have CET/IBT enforced,
LLVM makes that direct branch legal by placing an endbr64 immediately
after the callq.  Where I was placing the ret-clean sequence... this blows
up badly, in unhibernate / resume situations.

In the Functions marked exposesReturnsTwice(), skip doing ret-clean.
(placing the ret-clear after that endbr64 is much more difficult)
observed by mglocker, diagnosed by mlarkin, kettenis, guenther.