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
Fix -Xjitthreshold (for real this time).

My previous "fix" (c89d83e1c05979b68037ad15413fa4460a88e36f) had the
conditions reversed, so you _had_ to use -Xjitthreshold to get a non-zero
threshold, but when you did, you'd get the default instead of what you
asked for!

This was spotted by the jank tests.

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