writeThreadRegisters in ExecutionContext has no atomic behaviour

[rottaran]

All action should check for correctness and, then, either fail or finish completely. writeThreadRegisters fails to set the FS and GS base addresses if they are non-canonical. However, at the point where this check is done, all other registers are already touched. And then the EC gets into trapped state whereas the invocation returns failure to the user. No failure to carry out an action should ever cause side effects.