Allow the userspace ID register fields to be read from the kernel

To allow consistent values to be used in both the kernel and userspace
create a function for these to be read from the kernel. They use a newly
created macro with the name of the ID register to read. For now there is
redundant information in the user_regs array as it still holds the CRm and
Op2 values, however this will be fixed in a later change.

This will be used by ptrace to allow hardware breakpoints in userspace.

Sponsored by: DARPA, AFRL