Enable stack-allocatable-closures on ARM.
Implement stack-allocatable-vectors on ARM.
Fix allocation of large objects on ARM.

Yet another immediate encoding issue.
Adjust dynamic-extent.impure tests.

Remove all instance of :fails-on ppc, they have been passing for quite
some time already.

Fix :recheck-nested-dx-bug test on non-x86oids, since they can't
dx-allocate unboxed arrays.