AArch64: oat patches should be 32-bit ints.

This makes the arm64 backend consistent with the behaviour of the code
in oat_writer.cc and in the patchoat tool.
It also reduces the size of boot.oat by 1.6% (aosp_arm64-eng build).

Change-Id: Ia0b96737159c08955cd7b776ee396ff578cd58f6