Add implementations of __readfs{byte,word,dword,qword} to Intrin.h

Differential Revision: http://llvm-reviews.chandlerc.com/D2606

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199958 91177308-0d34-0410-b5e6-96231b3b80d8