Add support for the X86 secure guard extensions instructions in assembler (SGX).

This allows assembling the two new instructions, encls and enclu for the
SKX processor model.

Note the diffs are a bigger than what might think, but to fit the new
MRM_CF and MRM_D7 in things in the right places things had to be
renumbered and shuffled down causing a bit more diffs.

rdar://16228228