gdb: revert local modification to handle wchar_t

gdb: revert local modification to handle wchar_t

This local modification has been a real pain to maintain as each version
of gdb modifies charset.c to the point it doesn't merge cleanly.  Now
that the base iconv has been updated, the original gdb code should work
fine.  The config header has also been updated to reflect the new iconv
capabilities.