riscv64: Add extension restrictions to assembler. am: a24ffcd413 am: 04f74e35e3

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

Change-Id: I5d0f2a0603e2d1595d5d7dabc953ad358f4bf80b
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>