Add riscv64 support am: 7aa57c1135

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

Change-Id: Ic61d4c01d193f887ef19e3177bb084ca954f72d7
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>