cmd/link: put zero-sized data symbols at same address as runtime.zerobase

cmd/link: put zero-sized data symbols at same address as runtime.zerobase

Put zero-sized data symbols at same address as runtime.zerobase,
so zero-sized global variables have the same address as zero-sized
allocations.

Change-Id: Ib3145dc1b663a9794dfabc0e6abd2384960f2c49
Reviewed-on: https://go-review.googlesource.com/c/go/+/490435
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>