Add riscv64 support am: 7aa57c1135 am: b082ab0ce6

Original change: https://android-review.googlesource.com/c/platform/external/libmpeg2/+/2168947

Change-Id: If5f3d0436553ccc4034ac21e842b912f50b5072a
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>