riscv64: add missing UNUSED annotations.

The code compiles on other architectures that implement JNI compiler,
because they use these variables. However the code won't compile for
RISC-V as it falls into the default (unsupported) case.

Test: lunch aosp_riscv64-userdebug && m dist

Change-Id: I16010e806fe6c51fb0a7a20111e0d1feefde018c
6 files changed