Extend Link() to allow linking to the default namespace.

The code path through native bridge and NDK translation should
propagate nullptr as well.

This to support a later change that changes the way native test
libraries are loaded in ART run tests.

Test: art/test/testrunner/testrunner.py --target --64 --optimizing
Bug: 130340935
Change-Id: I934d11942e41ccc6d140a7044faa160b006166f1
3 files changed