ART: Use optimizing compiler in image_test

Expose some compiler options so we can run the image test with
the optimizing compiler without running into a stack overflow.

Also allow a variable amount of threads in CreateCompilerDriver.
Use 16 as a middle ground on the host to speed up the otherwise
now slowed-down test.

Bug: 27240085
Bug: 27552475
Change-Id: I8db5055d32ae722c8f430903244faa9166cc4886