c++: Make references to __cxa_pure_virtual weak.

If a program has no other dependencies on libstdc++, we shouldn't require it
just for __cxa_pure_virtual, which is only there to give a prettier
diagnostic before crashing the program; resolving the reference to NULL will
also crash, just without the diagnostic.

gcc/cp/ChangeLog
2020-05-11  Jason Merrill  <jason@redhat.com>

	* decl.c (cxx_init_decl_processing): Call declare_weak for
	__cxa_pure_virtual.