Add MINGW64 support on MSYS2 platform

[gokhanettin]

It seems we can not build with x86_64-w64-mingw32-gcc.exe on MSYS2 due
to the fact that

• sizeof(long) is different on general UNIX and MINGW64, 8 and 4,
  respectively.
• Assumption on MSVC compiler if sys.platform is win32 in setup.py.

Using long long instead of long and handling GCC cases on win32
platforms seem to be enough for MINGW64 support without breaking
anything.

Credit goes to http://proglab.blog.fc2.com/blog-entry-48.html and Google
Translate.

[snaury]

Thank you for the patch, however I'm afraid this patch may be incorrect, even if it happens to work for you in some cases. As far as I remember, win64 has ABI that is very different to other operating systems, so using switch_amd64_unix.h on win64 feels wrong. For some reference, compare which registers are saved in boost.context between platforms:

• x86_64 unix: https://github.com/boostorg/context/blob/9db16049231e63d98e69f9187e31d2ed02ff758f/src/asm/jump_x86_64_sysv_elf_gas.S#L8
• x86_64 windows: https://github.com/boostorg/context/blob/9db16049231e63d98e69f9187e31d2ed02ff758f/src/asm/jump_x86_64_ms_pe_masm.asm#L7

You would notice, that windows requires xmm6-xmm15, as well as rsi and rdi to be saved.

I vaguely remember that it was actually possible to use platform/switch_x64_masm.obj with mingw64, although to be honest I haven't been developing on Windows for almost 4 years and might be mistaken.

[gokhanettin]

I see. platform/switch_x64_masm.obj makes more sense. Thanks for the detailed explanation. I will send another PR for it.