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>