Qemu: Use all CPU cores am: 7a490b995a

Original change: https://android-review.googlesource.com/c/platform/art/+/2984614

Change-Id: I536c03a909173e09bd499f1d684fe55be87e9392
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>