Targets.cpp: [cygwin] Add the predefined macro "_X86_", according to newer...

Targets.cpp: [cygwin] Add the predefined macro "_X86_", according to newer version of cygwin/w32api.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@170188 91177308-0d34-0410-b5e6-96231b3b80d8