Fix -Xjitthreshold.

Previously, we'd always overwrite the user-supplied value because
the architecture-specific default gets set so late.

Bug: https://code.google.com/p/android/issues/detail?id=52017
Change-Id: I469bf9ce599820f5ce3dea346aa8f680deffb0c5

Bug: 8285558
Bug: https://code.google.com/p/android/issues/detail?id=52017
Change-Id: I28270f2573d46929eb10d30789fecf7d5a8cea75