Add parallel build support for perf tests.

Add parallel build support for perf tests.

gdb/testsuite/ChangeLog:

	* Makefile.in (workers/%.worker, build-perf): New rule.
	(GDB_PERFTEST_MODE): New variable.
	(check-perf): Use it.
	(clean): Clean up gdb.perf parallel build subdirs.
	* lib/build-piece.exp: New file.
	* lib/gdb.exp (make_gdb_parallel_path): New function
	(standard_output_file, standard_temp_file): Call it.
	(GDB_PARALLEL handling): Make outputs,temp,cache directories as subdirs
	of $GDB_PARALLEL.
	* lib/cache.exp (gdb_do_cache): Call make_gdb_parallel_path.