aix: fix ASM_OUTPUT_DEF warning

aix: fix ASM_OUTPUT_DEF warning

ASM_OUTPUT_DEF is used by GCC as a proxy to enable alias support.  But
the macro does not provide the necessary implementation for symbol aliasing
on AIX, which uses a different implementation.  On AIX, the macro was empty.
Because the macro is empty, none of the macro argument are used, which
means that some of the variables in the function that calls the macro are
never used.  This causes a warning when building varasm.c, and -Werror
converts that into a fatal error.

This patch references the macro arguments as (void) to prevent the
warning.

2020-12-05  Rainer Orth  <ro@CeBiTec.Uni-Bielefeld.DE>
	    David Edelsohn  <dje.gcc@gmail.com>

gcc/ChangeLog:

	* config/rs6000/xcoff.h (ASM_OUTPUT_DEF): Reference macro arguments.