x86-64: also diagnose far returns / IRET with ambiguous operand size

x86-64: also diagnose far returns / IRET with ambiguous operand size

Other than near returns these default to 32-bit operand size, and hence
it isn't really unlikely that 64-bit forms are meant. Hence these should
have disambiguating suffixes. In Intel mode, however, don't error in
these cases unconditionally - MASM accepts these without suffix _and_
without warning.