[AArch64] Replace some uses of report_fatal_error with reportError in AArch64 ELF object writer

If we can't handle a relocation type, report it as an error in the source,
rather than asserting. I've added a more descriptive message and a test for the
only cases of this that I've been able to trigger.

Differential Revision: http://reviews.llvm.org/D18388

llvm-svn: 264156