x86: Intel syntax implies Intel mnemonics

x86: Intel syntax implies Intel mnemonics

As noted in the context of d53e6b98a259 ("x86/Intel: correct disassembly
of fsub*/fdiv*") there's no such thing as Intel syntax without Intel
mnemonics. Enforce this on the assembler side, and disentangle command
line option handling on the disassembler side accordingly.

As a result in the opcode table specifying ATTMnemonic|ATTSyntax becomes
redundant with just ATTMnemonic. Drop the now meaningless ATTSyntax and
remove the then no longer accessible templates.