Remove --native-debuggable compiler option.

Check if both --debuggable and --generate-debug-info are set instead.

History: I have recently added the --native-debuggable flag to control
whether extra stack maps are generated in order to produce accurate
native line number mapping of all generated code. I didn't want to
include it into --debuggable since we compile boot.oat as debuggable
and so it would be affected.

On second thought, it would have been reasonable to generate the extra
stackmaps only if both --debuggable and --generate-debug-info are set,
instead of introducing another compiler flag. This means we do not
affect boot.oat but we still get the extra stack maps if DWARF debug
information is explicitly requested.

Change-Id: I7e5e557e4850a88b3b6f86178d2cb645fb1e1110