riscv64: hand-code trampoline instructions.

We don't have a macro-assembler for riscv64 yet, but we need to
generate trampolines for boot images and encode them in the .oat files.

Bug: b/271573990
Test: cuttlefish boots and uses dexpreopt artifacts
      $ lunch aosp_cf_riscv64_phone-userdebug && m
      $ launch_cvd --gpu_mode=drm_virgl
      # observe no crashes in zygote
      # observe that.vdex files are used in logcat
Change-Id: I605e93374e70cde29cdf893b14210f0c60d74129