Remove cmpxchg and cmpxchg64 syscalls

[michaeljclark]

cmpxchg and cmpxchg64 syscalls have been removed from the Linux ABI as the A extension is now mandatory for the Linux/UNIX platform. They can be removed from target-riscv/user_syscall.c. See https://github.com/riscv/riscv-qemu/blob/07146abe22e5ea5948fda5d511fd1fd228e196fa/target-riscv/user_syscall.c

[michaeljclark]

The https://github.com/riscv/riscv-qemu/tree/qemu-upstream-v1 branch removes the cmpxchg syscalls

We can keep this issue open until we rebase master against upstream or backport the change.

[michaeljclark]

The cmpxchg syscalls have been removed in the riscv-next branch