x86: adjust disassembly of insns operating on selector values

Bring disassembly back in line with what the assembler accepts, thus
also making it self-consistent (with, in particular selector load/store
insns). While there further add D to all affected insns except ARPL
(where S is used, matching LAR/LSL), to also behave correctly in suffix-
always mode.

While there also hook up the Intel variant of the LKGS test.