Instructions from atomic extension are not supported

[talubik]

Hello. I was experimenting with a library on OpenCL and encountered a problem with atomic operations. Everything works on simx, but not works on the other drivers.

I discovered that support for the RISC-V atomic extension is declared, so I wrote a test using the amoadd instruction. It passes on simx, but on rtlsim it produces an incorrect result, and in debug mode it throws an error during the fetch instruction.