riscv64: Make "C" extension default for assembler tests. am: b9e18d2ed0

Original change: https://android-review.googlesource.com/c/platform/art/+/2971995

Change-Id: If3d958005a6eed0d9f95808d1fe0a59bb10a8f27
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>