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.
MS intrinsics: implement __readgs{byte,word,dword,qword} (PR19054)