YASM requires a debug format to be specified with -g. Currently the
only combination that I can make work at all is DWARF-2/ELF (YASM
doesn't support Mach-O debugging at all, and its support for CV8/MSVC
and MinGW/DWARF-2 appears to be broken), so debugging is only enabled
automatically for ELF at the moment. For other formats, we don't
specify -g at all, which is how the old build system behaved.