Support A extension

[mohanson]

trafficstars

We can implement the A extension on ckb-vm: considering that some Rust codes may use atomic operations, and rust only provides riscv64imac-unknown-none-elf target.

$ rustup target list | grep riscv
riscv32i-unknown-none-elf
riscv32imac-unknown-none-elf
riscv32imc-unknown-none-elf
riscv64gc-unknown-linux-gnu
riscv64gc-unknown-none-elf
riscv64imac-unknown-none-elf