x86-64: accept 64-bit LFS/LGS/LSS forms with suffix or operand size specifier

x86-64: accept 64-bit LFS/LGS/LSS forms with suffix or operand size specifier

Since we accept these without suffix / operand size specifier, we should
also do so with one. (The fact that we unilaterally accept these, other
than far branches, rather than limiting them to Intel64 mode, will be
taken care of later on.)

Also take the opportunity and make sure "lfs <reg>, tbyte ptr <mem>"
et al get rejected outside of 64-bit mode. This became broken by
dc2be329b950 ("i386: Only check suffix in instruction mnemonic").
Furthermore cover lgdt et al in the Intel syntax handling as well, which
continued to work after said commit just by coincidence.