For generic functions, we have to leave the builtins in OCALL form,
rather than transform to specific ops, since we don't know the exact
types involved. Allow export/import of builtins in OCALL form.
Added new export/import test mapimp.go.
Change-Id: I571f8eeaa13b4f69389dbdb9afb6cc61924b9bf2
Reviewed-on: https://go-review.googlesource.com/c/go/+/321750
Trust: Dan Scales <danscales@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
