KVM: x86: Move inlined GPR, CR, and DR helpers from x86.h to regs.h

KVM: x86: Move inlined GPR, CR, and DR helpers from x86.h to regs.h

Move inlined General Purpose Register, Control Register, and Debug
Register helpers from x86.h to the aptly named regs.h, to help trim
down x86.h (and x86.c in the future).

Move *very* select EFER functionality as well, but leave behind the bulk of
EFER handling and all other MSR handling.  There is more than enough MSR
code to carve out msrs.{c,h} in the future.  Give is_long_bit_mode()
special treatment as it's more along the lines of a CR4 bit check, but just
happens to be accessed through an MSR interface.  And more importantly,
because giving regs.h access to is_long_bit_mode() greatly simplifies
dependency chains.

No functional change intended.

Reviewed-by: Yosry Ahmed <yosry@kernel.org>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Link: https://patch.msgid.link/20260529222223.870923-8-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>