tool/gocross: add debug data to CGO builds

We don't build a lot of tools with CGO, but we do build some, and it's
extremely valuable for production services in particular to have symbols
included - for perf and so on.

I tested various other builds that could be affected negatively, in
particular macOS/iOS, but those use split-dwarf already as part of their
build path, and Android which does not currently use gocross.

One binary which is normally 120mb only grew to 123mb, so the trade-off
is definitely worthwhile in context.

Updates tailscale/corp#20296

Signed-off-by: James Tucker <james@tailscale.com>

tool/gocross/autoflags.go

@@ -36,7 +36,7 @@ func autoflagsForTest(argv []string, env *Environment, goroot, nativeGOOS, nativ
 		targetOS    = cmp.Or(env.Get("GOOS", ""), nativeGOOS)
 		targetArch  = cmp.Or(env.Get("GOARCH", ""), nativeGOARCH)
 		buildFlags  = []string{"-trimpath"}
-		cgoCflags   = []string{"-O3", "-std=gnu11"}
+		cgoCflags   = []string{"-O3", "-std=gnu11", "-g"}
 		cgoLdflags  []string
 		ldflags     []string
 		tags        = []string{"tailscale_go"}