ART: Fix Mac-specific things in x86-64 assembly

We incorrectly used double-underscore on x86-64 as VAR (used by
trampoline macros) introduced an additional underscore. Add CALLVAR
just as on x86.

Bug: 30212852
Change-Id: I2a95dd736453160769ea1a8b24c66f1738049ccb
Test: m test-art-host (Linux)
Test: builds on Mac