Target: invert condition for Windows

The Microsoft ABI and MSVCRT are considered the canonical C runtime and ABI.
The long double routines are not part of this environment.  However, cygwin and
MinGW both provide supplementary implementations.  Change the condition to
reflect this reality.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213896 91177308-0d34-0410-b5e6-96231b3b80d8