[OS/2] Redo debug symbol file generation.

When --enable-debug-symbols is ON, the makefiles will now generate
.DBG files with HLL debug info (NB04) per each .EXE and .DLL for
supported linkers (WLINK). Otherwise, .XQS will be generated from
.MAP (requires mapxqs from EXCEPTQ).
[OS/2] Disable multiple make jobs when building NSS.

This attempts to fix kbuild make crashes in -jN mode where N > 1.