Default generated plugin should set "a" flag in isa if "A" in ispec

[Elizafox]

"A" in ispec signals atomic instructions are present. If this flag is present in ispec, isa must have the "a" flag or it will not work.

This should be a relatively simple change. I was bitten by this issue recently with an ISA file from riscv-config. It was non-obvious to me at first until I looked at the plugin.