Add parallel build support for perf tests.

Add parallel build support for perf tests.

gdb/testsuite/ChangeLog:

	* Makefile.in (check/%.exp): Pass directory for GDB_PARALLEL.
	(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/cache.exp (gdb_do_cache): Include $GDB_PARALLEL in path name.
	* lib/gdb.exp (standard_output_file): Include $GDB_PARALLEL in path
	name.
	(standard_temp_file): Ditto.
	(GDB_PARALLEL handling): Make outputs,temp,cache directories as subdirs
	of $GDB_PARALLEL.