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.

llvm-svn: 218389