[PATCH v2] mscoree: Implement VTable fixup for x86_64 architecture.

[Vincent Povirk]

So, I'm trying to understand which registers we need to preserve, and
I found some pages on MSDN.

register usage: https://msdn.microsoft.com/en-us/library/9z1stfyw.aspx
parameter passing: https://msdn.microsoft.com/en-us/library/zthk2dkh.aspx

I think we need to preserve all parameter registers and can freely use
any scratch registers that are not parameter registers, and that we
only need to worry about the 64-bit Windows ABI.

MSDN seems to say that only XMM0 through XMM3 are used for parameter
passing, and that the extra bits in YMM0 through YMM3 are scratch
registers. I think that means we can ignore XMM4, XMM5, and AVX
extensions.

The requirements may be different for the non-Windows ABI, but I hope
we can trust the compiler to take care of that as long as we're not
calling such a function directly.

I'm not sure if we can use __attribute__((force_align_arg_pointer))
directly in this way, but I will defer to others on that question.