cmd/link: move cgo export map from loadcgo to setCgoAttr

Currently, both loadcgo and setCgoAttr do some processing of
cgo_export_static and cgo_export_dynamic cgo directives, which means
they both have to parse them. There's no reason to do this in loadcgo,
so move all directive processing to setCgoAttr.

For #40724.

Change-Id: Icb3cdf7ef3517e866dd220e40a5f5dec7fd47e2b
Reviewed-on: https://go-review.googlesource.com/c/go/+/309339


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