Fix bug with idx() for the HHVM open source build

Summary: An assertion (func != nullptr) triggers on the HHVM OSS build when idx() is
not defined and an <?hh file tries to call idx().

This diff fixes the issue by updating the bytecode emitter so that it only
emits the Idx instruction if systemlib defines idx().

Reviewed By: @sgolemon

Differential Revision: D1474391