cmd/compile/internal/gc: make embedded unexported structs RO

cmd/compile/internal/gc: make embedded unexported structs RO

gc will need to be rebuild.

Package that assume f.PkgPath != nil means a field is unexported and
must be ignored must be revised to check for
	f.PkgPath != nil && !f.Anonymous,
so that they do try to walk into the embedded fields to look for
exported fields contained within.

Closes #12367, fixes #7363, fixes #11007, and fixes #7247.

Change-Id: I16402ee21ccfede80f277f84b3995cf26e97433d
Reviewed-on: https://go-review.googlesource.com/14085
Reviewed-by: Russ Cox <rsc@golang.org>