Normally for 'performance', nptl uses the _Unwind_ForcedUnwind() function,
which is provided by GCC.  However, this function depends on libunwind tracking
all register state, which neither libunwind itself (which we do not want as a
core dependency) or GCC's libunwind support register state on some newer x86
CPUs fully.

Thusly, we turn off the unwind-using code and return to using the traditional
cleanup functions.