The trampoline size is part of the ABI, so it cannot change. Move the
logic from the stdcall and thiscall trampolines to the functions they
call, to reduce them both to 10 bytes.
This drops the previously added support for raw THISCALL closures on
non-Windows. (Non-raw THISCALL closures still work.)
Linux supports the stdcall calling convention, either via functions
explicitly declared with the stdcall attribute, or via code compiled
with -mrtd which effectively makes stdcall the default.
This introduces FFI_STDCALL, FFI_THISCALL, and FFI_FASTCALL on
non-Windows x86-32 platforms, as non-default calling conventions.
