cmd/link: add debug print in deadcode

This matches the prints that deadcode prints later
as the algorithm progresses under -v=2.
It helps to see the initial conditions with -v=2 as well.

Change-Id: I06ae86fe9bd8314d003148f3d941832c9b10aef1
Reviewed-on: https://go-review.googlesource.com/c/go/+/288817
Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/link/internal/ld/deadcode.go

@@ -91,6 +91,10 @@ func (d *deadcodePass) init() {
 		names = append(names, exp)
 	}
 
+	if d.ctxt.Debugvlog > 1 {
+		d.ctxt.Logf("deadcode start names: %v\n", names)
+	}
+
 	for _, name := range names {
 		// Mark symbol as a data/ABI0 symbol.
 		d.mark(d.ldr.Lookup(name, 0), 0)