target/102348 - fix powerpc-lynxos build

This fixes a similar issue for powerpc-lynxos as fixed for i686-lynxos
already.

2021-09-15  Richard Biener  <rguenther@suse.de>

	PR target/102348
	* config/rs6000/lynx.h: Remove undef of PREFERRED_DEBUGGING_TYPE
	to inherit from elfos.h

gcc/config/rs6000/lynx.h

@@ -80,7 +80,6 @@
 
 #undef SIZE_TYPE
 #undef ASM_OUTPUT_ALIGN
-#undef PREFERRED_DEBUGGING_TYPE
 
 /* The file rs6000.c defines TARGET_HAVE_TLS unconditionally to the
    value of HAVE_AS_TLS.  HAVE_AS_TLS is true as gas support for TLS