Add a64 command.
!a64 - Input x64 asm code.
Since the a command does not support 64-bit instruction mnemonics on windbg:
0:000> a .
00000000`77ae7980 xor rax,rax
xor rax,rax
^ Couldn't resolve 'xor rax,rax'
I import XED to support 64-bit instruction mnemonics:
0:000> !a64 .
0000000077ae7980 xor rax,rax
xor rax,rax = 48 31 c0