Add -z separate-code to default ARM64 device flags

Removing XOM had the side effect of removing "-z separate-code", which
was needed to override a new default after a recent toolchain update.
This led to some performance regressions in some tests. For now, add
this flag to the global arm64 device flags to return to the previous
behavior.

Bug: 150506341
Test: m -j
Change-Id: I54787954141d133e245dfd259a37bf4c3c8e7caa