cmd/link: zero symtab fields correctly

CL 69370 introduced a hasmain field to moduledata after the
modulehashes slice. However that code was relying on the zeroing
code after it to cover modulehashes if len(Shlibs) == 0. The
hasmain field gets in the way of that. So clear modulehashes
explicitly in that case.

Found when looking at #22250. Not sure if it's related.

Change-Id: I81050cb4554cd49e9f245d261ef422f97d026df4
Reviewed-on: https://go-review.googlesource.com/70730
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

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

@@ -639,6 +639,12 @@ func (ctxt *Link) symtab() {
 		moduledata.AddAddr(ctxt.Arch, modulehashes)
 		moduledata.AddUint(ctxt.Arch, uint64(len(ctxt.Shlibs)))
 		moduledata.AddUint(ctxt.Arch, uint64(len(ctxt.Shlibs)))
+	} else {
+		moduledata.AddUint(ctxt.Arch, 0) // modulename
+		moduledata.AddUint(ctxt.Arch, 0)
+		moduledata.AddUint(ctxt.Arch, 0) // moduleshashes slice
+		moduledata.AddUint(ctxt.Arch, 0)
+		moduledata.AddUint(ctxt.Arch, 0)
 	}
 
 	hasmain := ctxt.BuildMode == BuildModeExe || ctxt.BuildMode == BuildModePIE