unwind-dw2-fde.c change breaks AIX

[David Edelsohn]

	I am testing with it defined to "".

	USER_LABEL_PREFIX originally was defined in rs6000.h by J"orn in
Sep 1996 (FSFChangeLog.11).  Maybe he has some idea why he chose that
definition.

	Looking at the ChangeLog entry, J"orn refers to setting the
USER_LABEL_PREFIX macro instead of ASM_OUTPUT_LABELREF.  aix.h defines
both, so it seems moot.  ASM_OUTPUT_LABELREF probably was defined because
USER_LABEL_PREFIX macro was doing the wrong thing :^).

	I am not sure whether it is worth removing the redundant
definition of ASM_OUTPUT_LABELREF.  I'll probably do that on the trunk. 

David