Micro-optimize calling alloc_tramp on x86_64.

In allocation-tramp, instead of using R11 temp-reg for storing the
address of alloc_tramp, use alloc-tn, because the result of
alloc_tramp will be stored there and its value doesn't have to be
preserved. And most of the time it will be allocated to a
non-64-bit-mode register, allowing to save 2 bytes on encoding.