[dev.typeparams] cmd/compile: add import/export of calls to builtin functions

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>