cmd/link: handle dynamic import variables on Darwin in plugin mode
CL 501855 added support for cgo_dynamic_import variables on Darwin.
But it didn't support the plugin build mode on amd64, where the
assembler turns a direct load (R_PCREL) to a load via GOT
(R_GOTPCREL). This CL adds the support. We just need to handle
external linking mode, as this can only occur in plugin or shared
build mode, which requires external linking.
Fixes #67976.
Updates #50891.
Change-Id: I0f56265d50bfcb36047fa5538ad7a5ec77e7ef96
Reviewed-on: https://go-review.googlesource.com/c/go/+/592499 Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>