Add riscv64 support am: 7aa57c1135 am: b082ab0ce6 am: 3f09b52eba

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

Change-Id: I9d65d0a7d6cb16662d36a80b25066025ae92fd2a
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>