[dev.link] cmd/link: don't set unreachable string variables in addstrdata
[dev.link] cmd/link: don't set unreachable string variables in addstrdata

If the variable is not reachable, don't bother setting it.

The old behavior was to set it but not mark it reachable, nor the
string data it points to. This was changed in CL 219226, which
changed the variable and the string data to always reachable.
Typically it shouldn't matter much besides some waste of binary
size. But it does matter on AIX (crash in make.bash). I haven't
looked into why.

Fix AIX build.

Change-Id: I546a0c94ad77b10485ceb66e1288a408e2a2a3e2
Reviewed-on: https://go-review.googlesource.com/c/go/+/223380
Run-TryBot: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Jeremy Faller <b3f594e10a9edcf5413cf1190121d45078c62290@golang.org>
Reviewed-by: Than McIntosh <4b2922593166fc595a9b1a408f34f5d3817fe9d2@google.com>

CAFxX/go,nawawi/go,akutz/go,CAFxX/go,CAFxX/go,nawawi/go,golang/go,golang/go,akutz/go,cloudflare/go,cloudflare/go,nawawi/go,golang/go,nawawi/go,akutz/go,CAFxX/go,akutz/go,CAFxX/go,golang/go,cloudflare/go,nawawi/go,CAFxX/go,nawawi/go,akutz/go,cloudflare/go,golang/go,cloudflare/go,CAFxX/go,nawawi/go,golang/go,akutz/go,cloudflare/go,golang/go,akutz/go,nawawi/go,cloudflare/go,cloudflare/go,golang/go,akutz/go,CAFxX/go