intrin.h: Fix up bugs in the cr3 and msr intrinsics

Don't include input and output regs in clobbers.  Prefix some
identifiers with __.  Add a memory constraint to __readcr3 to prevent
reordering.  This constraint is heavy handed, but conservatively
correct.

Thanks to PaX Team for the suggestions.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205778 91177308-0d34-0410-b5e6-96231b3b80d8