libthr: Create landingpad and call dtor in thr_once in C++ exception

[aokblast]

Summary

In _thr_once, if init_routine fails with exception, the stack allocate
__cup will become invalid if unwinder walk back and free the stack of
thr_once. This cause cleanup linked list be dangling. In here, we give
the cleanup function a landing pad so that we td can correctly point
to previous cleanup function.

[kib]

Does this need to be conditional on _PTHREAD_FORCED_UNWIND (I am not sure)?

[aokblast]

In D61212#1403439, @kib wrote:

Does this need to be conditional on _PTHREAD_FORCED_UNWIND (I am not sure)?

Without unwinding, we actually calls no landing pad as in !_PTHREAD_FORCED_UNWIND case we assume that no landing pad happens. We can definiately add _PTHREAD_FORCED_UNWIND guard in here to save some bytes, but I think it is much clear now as a macro.