Remove unused but set variables.

Reported by clang13 -Wunused-but-set-variable:

gcc/ChangeLog:

	* dbgcnt.c (dbg_cnt_process_opt): Remove unused but set variable.
	* gcov.c (get_cycles_count): Likewise.
	* lto-compress.c (lto_compression_zlib): Likewise.
	(lto_uncompression_zlib): Likewise.
	* targhooks.c (default_pch_valid_p): Likewise.

libcpp/ChangeLog:

	* charset.c (convert_oct): Remove unused but set variable.