PATCH: PR gas/1874: mwait/monitor don't work in 64bit

[Jan Beulich]

>Are you saying "monitor %eax,%ecx,%edx" should be allowed in 64bit
>mode? Why? How will it be used?

Just like any other 32-bit addressing in 64-bit mode: probably rarely,
but it should be allowed for whoever is able to find meaningful use for
this. We also don't disallow 16-bit addressing in 32-bit mode, even
though it's unlikely to ever be used...

Jan