[X86] Fix the implementation of __readcr3/__writecr3 to work in 64-bit mode

We need to use a 64-bit type in 64-bit mode so a 64-bit register
will get used in the generated assembly. I've also changed the
constraints to just use "r" intead of "q". "q" forces to a only
an a/b/c/d register in 32-bit mode, but I see no reason that
would matter here.

Fixes Nico's note in PR19301 over 4 years ago.

Differential Revision: https://reviews.llvm.org/D70101