[Bug libstdc++/85813] make_exception_ptr should support __cxa_init_primary_exception path even with -fno-exceptions

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85813

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
As you pointed out in PR 85812 the constructor of the exception can throw. That
can happen even if the current translation unit is not built with exceptions,
if the constructor is defined in a different translation unit.

Since make_exception_ptr is noexcept, we simply avoid the possibility of
termination by returning an empty exception_ptr.