Quick fix: Triple::isOSMSVCRT() should be false for targeting cygwin.

It affected callee's stack pop in x86. It is one of devergences between cygwin and mingw since mingw-gcc-4.6.

Added testcases to llvm/test/CodeGen/X86/win32_sret.ll for cygwin.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205688 91177308-0d34-0410-b5e6-96231b3b80d8