CodeGen: add __readfsdword builtin

The Windows NT SDK uses __readfsdword and declares it as a compiler provided
builtin (#pragma intrinsic(__readfsword).  Because intrin.h is not referenced
by winnt.h, it is not possible to provide an out-of-line definition for the
intrinsic.  Provide a proper compiler builtin definition.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@220859 91177308-0d34-0410-b5e6-96231b3b80d8