Fix ppc64 ELFv1 assertion failure

Fix ppc64 ELFv1 assertion failure

Bogus assembly can hit an assertion in opd_entry_value when the symbol
referenced by a function descriptor is undefined.  Worse, the code
after the assert copies unitialised memory to return the code section.
This uninitialised pointer can later be dereferencd, possibly causing
a linker segmentation fault.

	* elf64-ppc.c (opd_entry_value): Remove assertion.  Instead,
	return -1 if symbol referenced is not defined.  Tidy.