cmd/go: remove -w workaround for -buildmode=plugin on Darwin

cmd/go: remove -w workaround for -buildmode=plugin on Darwin

The problem causing the assert in #21647 are fixed at this point,
along with various other linker issues with plugin + Darwin. With
this in mind, remove the "-ldflags=-w" workaround for plugin mode on
Darwin and re-enable the appropriate tests misc/cgo/testplugin

Fixes #21647.
Fixes #27502.

Change-Id: I5b662987b138b06cfc9e1f9f6d804cf682bd501a
Reviewed-on: https://go-review.googlesource.com/c/go/+/206198
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>