Add support for stdcall, thiscall, and fastcall on non-Windows x86-32

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.
Merge pull request #85 from joshtriplett/stdcall

stdcall support on Linux