cmd/link: only create .init_array section if not empty

This avoids a warning from old versions of the GNU linker or glibc.

No test because these old versions are not readily available.
I tested this by hand on CentOS 6.

Fixes #28722

Change-Id: I16640c9b83a79f759ec68fac64874803e74fbbfb
Reviewed-on: https://go-review.googlesource.com/c/153257
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -1313,7 +1313,7 @@ func (ctxt *Link) dodata() {
 	case BuildModeCArchive, BuildModeCShared, BuildModeShared, BuildModePlugin:
 		hasinitarr = true
 	}
-	if hasinitarr {
+	if hasinitarr && len(data[sym.SINITARR]) > 0 {
 		sect := addsection(ctxt.Arch, &Segdata, ".init_array", 06)
 		sect.Align = dataMaxAlign[sym.SINITARR]
 		datsize = Rnd(datsize, int64(sect.Align))