X86Subtarget.h: Recognize x86_64-cygwin.

In the LLVM side, x86_64-cygwin is almost as same as x86_64-mingw32.
X86JITInfo.cpp: Apply x64 version of X86CompilationCallback() to Cygwin64.

For now, (defined(X86_64_JIT) && defined(__CYGWIN__)) satisfies Cygwin64.