cmd/link: limit darwin dynlink symbol exports

The pclntable contains pointers to functions. If the function symbol
is exported in a plugin, and there is a matching symbol in the host
binary, then the pclntable of a plugin ends up pointing at the
function in the host module.

This doesn't work because the traceback code expects the pointer to
be in the same module space as the PC value.

So don't export functions that might overlap with the host binary.
This way the pointer stays in its module.

Updates #18190

Change-Id: Ifb77605b35fb0a1e7edeecfd22b1e335ed4bb392
Reviewed-on: https://go-review.googlesource.com/34196


Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>