Refactor the RelocVisitor::visit method

This change replaces the brittle if/else chain of string comparisons
with a switch statement on the detected target triple, removing the
need for testing arbitrary architecture names returned from
getFileFormatName, whose primary purpose seems to be for display
(user-interface) purposes. The visitor now takes a reference to the
object file, rather than its arbitrary file format name to figure out
whether the file is a 32 or 64-bit object file and what the detected
target triple is.

A set of tests have been added to help show that the refactoring processes
relocations for the same targets as the original code.

Patch by Charlie Turner.
Add support for ARM and AArch64 BE object files

This change fixes the ARM and AArch64 relocation visitors in
RelocVisitor.  They were unconditionally assuming the object data are
little-endian.  Tests have been added to ensure that the
llvm-dwarfdump utility does not crash when processing big-endian
object files.

Patch by Charlie Turner.