Initial DWARF64 support for .debug_line

Some versions of the GNU MIPS toolchain generate 64-Bit DWARF (even though
it isn't really necessary).  This change adds support for the 64-Bit DWARF
format, but is not actually tested with >4GB of debug data.

Similar changes are in progress for llvm's version of DWARFDebugLine, in
review D1988.