cmd/compile: add -c flag

This will be used in the future to control backend concurrency.
See CL 40693.

In the meantime, make it a no-op.
This should fix the linux-amd64-racecompile builders.

Change-Id: Ibf3b2a7fff6f8f8c94f5fafb26e0500a51c8a4a6
Reviewed-on: https://go-review.googlesource.com/41614
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/gc/main.go

@@ -184,6 +184,8 @@ func Main(archInit func(*Arch)) {
 	objabi.Flagcount("W", "debug parse tree after type checking", &Debug['W'])
 	flag.StringVar(&asmhdr, "asmhdr", "", "write assembly header to `file`")
 	flag.StringVar(&buildid, "buildid", "", "record `id` as the build id in the export metadata")
+	var dashc int
+	flag.IntVar(&dashc, "c", 0, "makes -c work")
 	flag.BoolVar(&pure_go, "complete", false, "compiling complete package (no C or assembly)")
 	flag.StringVar(&debugstr, "d", "", "print debug information about items in `list`; try -d help")
 	flag.BoolVar(&flagDWARF, "dwarf", true, "generate DWARF symbols")