.travis.yml: Test on both 32-bit and 64-bit
prep_cif.c: Remove unnecessary ifdef for X86_WIN32

ffi_prep_cif_core had a special case for X86_WIN32, checking for
FFI_THISCALL in addition to the FFI_FIRST_ABI-to-FFI_LAST_ABI range
before returning FFI_BAD_ABI.  However, on X86_WIN32, FFI_THISCALL
already falls in that range, making the special case unnecessary.
Remove it.
Merge pull request #89 from joshtriplett/travis32

.travis.yml: Test on both 32-bit and 64-bit
Merge pull request #90 from joshtriplett/win32-unifdef

prep_cif.c: Remove unnecessary ifdef for X86_WIN32
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