x86: don't accept FI{LD,STP,STTP}LL in Intel syntax mode

x86: don't accept FI{LD,STP,STTP}LL in Intel syntax mode

As of commit dc2be329b950 ("i386: Only check suffix in instruction
mnemonic") these have been accepted even with "qword ptr" operand size
specifier, but in 64-bit mode they're now wrongly having a REX prefix
(with REX.W set) emitted in this case. These aren't Intel syntax
mnemonics, so rather than fixing code generation, let's simply reject
them. As a result, the Qword attribute can then be dropped, too.