Segmented Stack support for X64-MinGW

[Alex Crichton]

As part of the ongoing effort to upstream the patches that we're using
with the Rust language, this is a patch which adds support for
segmented stacks on 64-bit windows using MinGW. The original author
was @klutzy on GitHub [1], and I've just added some tests to it.

The implementation is the same as the 32-bit windows implementation by
assuming that the stack limit is in the TIB's arbitrary data slot [2].
The location of the TIB is slightly different, being pointed to by %gs
instead of %fs [3].

If anything else is required, please just let me know!

[1] - https://github.com/rust-lang/llvm/commit/12d885
[2] - http://en.wikipedia.org/wiki/Win32_Thread_Information_Block
[3] - http://en.wikipedia.org/wiki/X86-64
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