New codegen: allocate large objects using allocateLocal()

[simonmar]

See #3424.

In the new code generator, we should allocate large objects (larger than F * block size, for some suitable fraction F) using the RTS allocateLocal() API rather than from the nursery. It works to allocate them from the nursery -- this is what GHC 6.12 does after the fix in #3424 -- but then they will not be treated as large objects and will be copied during GC. Also, the allocation is likely to fail, requiring a trip through the RTS to put a large enough block in the nursery to satisfy the allocation.