riscv64: Make "C" extension default for assembler tests. am: b9e18d2ed0 am: 859452e0cc

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

Change-Id: Ifd605340d27d825781a687453e469e41544be684
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>