Document ABI conventions

[X512]

ABI conventions used by Haiku for each architecture (x86 gcc2, x86 gcc4+, x86_64) should be documented in some way (in web site, HaikuBook etc.).

At least following topics should be documented:

• Preserved registers, including SIMD registers etc.
• FPU state.
• Passing arguments to function and return value.
• Stack alignment including alignment origin.
• Exception handling and stack unwinding.
• Executable format.

If Haiku use some existing ABI, it should be referenced in documentation with Haiku specific changes, if any.

I tried to port some non C/C++ compiler (target ABI: x86 gcc4+) and make Haiku API bindings and experience various problems such as:

• Structs passed by value (such as pattern) are stored on stack as is, but classes such as BPoint are passed by pointer.
• StrokeLine(BPoint, BPoint, pattern) is not working if stack is not aligned.