Support _M_ARM / _M_ARM64?

[past-due]

Wanted to open this as an issue, although I'm afraid I can't be much help testing (yet).

Examples:
https://github.com/llvm-mirror/llvm/blob/master/lib/Support/Windows/Signals.inc
(Part 1, Part 2 Fixes)

It appears as though the proper offsets for filling the STACKFRAME64 struct for _M_ARM / _M_ARM64 are:

Arch	STACKFRAME64 Field	CONTEXT Value
Both	AddrPC.Offset	Pc
Both	AddrStack.Offset	Sp
_M_ARM64	AddrFrame.Offset	Fp
_M_ARM	AddrFrame.Offset	R11

And AddrModeFlat as the Mode