cmd/link: only dynamically export necessary symbols on darwin

Currently on darwin, when a symbol needs to be exported, we
export it both statically and dynamically. The dynamic export is
unnecessary for some symbols. Only export the necessary ones.

For special runtime C symbols (e.g. crosscall2), they used to be
exported dynamically, and we had a special case for pclntab to
not include those symbols (otherwise, when the dynamic linker
dedup them, the pclntab entries end up pointing out of the
module's address space). This CL changes it to not export those
symbols, and remove the special case.

Change-Id: I2ab40630742d48a09b86ee150aa5f1f7002b134d
Reviewed-on: https://go-review.googlesource.com/c/go/+/261497


Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jeremy Faller <jeremy@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>