Rename the marker used by Diff_Gcc_Nat.hs to ___ncg_debug_marker, to make
it clear it has nothing to do with the usual object-splitting machinery.

Improve Diff_Gcc_Nat:
* Emit a warning, and stop, if there are no debug markers in the NCG code.
* Handle .uahalf and .uaword, which appear in sparc assembly, but not x86.