MS intrinsics: don't declare __readeflags and __writeeflags in Intrin.h

They're already defined in ia32intrin.h, and this would cause including Intrin.h
in 64-bit mode to fail because of conflicting types. Update ms-intrin.cpp to
also run in 64-bit mode to catch things like this.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@203714 91177308-0d34-0410-b5e6-96231b3b80d8
MS intrinsics: implement __readgs{byte,word,dword,qword} (PR19054)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@203715 91177308-0d34-0410-b5e6-96231b3b80d8