add KMOW

[yjxxin]

Hi team

When we added the semantic function of AVX512 instructions, we found that remill did not implement the reading of the k register. For example, the following AVX512 instruction:

instruction	binary
kmovw   %ebx, %k1	c5 f8 92 cb

In the above instruction, the k1 register is used as the result operand, and it is also indicated in docs/XED/xed.txt that the K mask register needs to be used:

2630 KMOVW KMOVW_MASKmskw_MASKu16_AVX512 KMASK AVX512VEX AVX512F_KOP ATTRIBUTES: KMASK 2
0 REG0 EXPLICIT W NT_LOOKUP_FN I1 MASK_R
1 REG1 EXPLICIT R NT_LOOKUP_FN U16 MASK_B

But remill does not provide a method to read the K register.
To solve this problem, we need to add a variable to store the value of the K register in State.h, and add the size of the K register to the State variable.

I have submitted a PR and implemented the KMOVW instruction, hoping to provide valuable comments