libstdc++: Fix std::basic_stracktrace to not assume allocators throw std::bad_alloc

The standard allows allocators to throw any kind of exception, not only
something that can be caught as std::bad_alloc. std::basic_stracktrace
was assuming std::bad_alloc.

libstdc++-v3/ChangeLog:

	* include/std/stacktrace (basic_stacktrace::_Impl::_M_allocate):
	Do not assume allocators only throw std::bad_alloc.