riscv64: Add extension restrictions to assembler. am: a24ffcd413

Original change: https://android-review.googlesource.com/c/platform/art/+/2973813

Change-Id: I8303a57ffaab20090205492d1e93ea194e18fdd1
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>