Fix build fail with i686 toolchain

[volks73]

As noted in #31, I was experiencing a similar linker error and failure with the i686 toolchain. I had some time to dig into it a little bit and I believe I have a resolution for this issue that I would like to share.

The problem appeared to be the calling convention for the kernel32 DLL function and a u64 being used even with the i686 (32-bit) toolchain. Apparently on Windows, a DWORD is always 32-bit (u32) even on a 64-bit system (x86_64). The GetStdHandle declaration was using a u64.

Additionally, the system calling convention needed to be added because in 64-bit environments (x86_64), the calling convention is C but in 32-bit environments the calling convention is stdcall, as noted in the Rust book's FFI section:

Most of the abis in this list are self-explanatory, but the system abi may seem a little odd. This constraint selects whatever the appropriate ABI is for interoperating with the target's libraries. For example, on win32 with a x86 architecture, this means that the abi used would be stdcall. On x86_64, however, windows uses the C calling convention, so C would be used. This means that in our previous example, we could have used extern "system" { ... } to define a block for all windows systems, not only x86 ones.

The system calling convention automatically handles the difference depending on the environment/toolchain. Without this notation, the linker would fail because the calling convention would not match the declarations for all three functions.

After making these changes, I was able to successfully build the crate using the stable-i686-pc-windows-msvc toolchain with a x86 environment (Developer Prompt for x86) and pass all tests. I was also able to successfully build the crate with the stable-x86_64-pc-windows-msvc. I used the following commands from the x86 Developer Prompt on a 64-bit architecture running Windows 10 to build and pass the tests that were previously failing to build because of the linker error:

C:\set RUSTUP_TOOLCHAIN=stable-i686-pc-windows-msvc
C:\cargo test
...
test result: ok. 56 passed; 0 failed, 0 ignored; 0 measured; 0 filtered out
...
test result: ok. 18 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

[pravic]

What a mess with handle types.

[volks73]

As mentioned in #31, this pull request can be ignored.