[dev.link] cmd/link: fix pclntab symbol handling on AIX

[dev.link] cmd/link: fix pclntab symbol handling on AIX

On AIX, container symbols are handled in a weird way (unlike
other platforms): the outer symbol needs to have size (but still
no data), and the inner symbols must not be in the symbol table
(otherwise it overlaps with the outer symbol, which the system
linker doesn't like).

As of CL 241598, pclntab becomes a container symbol. We need to
follow the rule above for AIX.

Fix AIX build.

Change-Id: Ie2515a4cabbd8cf3f6d3868643a28f64ca3365a2
Reviewed-on: https://go-review.googlesource.com/c/go/+/246479
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jeremy Faller <jeremy@golang.org>