Filling in ARM64 ABI structs

[Martin Storsjö]

Hi,

I've noticed that some of the ABI specific structs for ARM64 are 
incomplete or missing - namely _JUMP_BUFFER in include/msvcrt/setjmp.h 
(not yet available) and _CONTEXT (not missing but only has got made-up 
content so far) and associated CONTEXT_* defines, and structs like 
RUNTIME_FUNCTION and UNWIND_HISTORY_TABLE in include/winnt.h.

As far as I know, these structs should be available in the win10 sdk. 
(MSDN doesn't have them but just says to refer to winnt.h for the CONTEXT 
struct at least). In order not to accidentally make myself uneligible for 
contributing code on these matters, I've tried to not look at the sdk.

Can someone help out with completeing these data structures according to 
whatever clean-room guidelines are required? When looking at existing code 
with git blame, they mostly appear as such without any further explanation 
on how they were derived, so I'm taking the more cautious route here.

// Martin