Minor updates for the x64 model.

• The decoder has been tweaked (mainly for use under SML). Previously it would succeed in decoding an instruction even if there were too few bytes.

• x64DisassembleLib has been removed. The function x64_disassemble has been implemented in x64AssemblerLib.

Add support for x86-64 assembly code.

Based on NASM syntax, as opposed to AT&T (which is the default for gas).