x86/asm: Move native_write_cr0/4() out of line

The pinning of sensitive CR0 and CR4 bits caused a boot crash when loading
the kvm_intel module on a kernel compiled with CONFIG_PARAVIRT=n.

The reason is that the static key which controls the pinning is marked RO
after init. The kvm_intel module contains a CR4 write which requires to
update the static key entry list. That obviously does not work when the key
is in a RO section.

With CONFIG_PARAVIRT enabled this does not happen because the CR4 write
uses the paravirt indirection and the actual write function is built in.

As the key is intended to be immutable after init, move
native_write_cr0/4() out of line.

While at it consolidate the update of the cr4 shadow variable and store the
value right away when the pinning is initialized on a booting CPU. No point
in reading it back 20 instructions later. This allows to confine the static
key and the pinning variable to cpu/common and allows to mark them static.

Fixes: 8dbec27a ("x86/asm: Pin sensitive CR0 bits")
Fixes: 873d50d5 ("x86/asm: Pin sensitive CR4 bits")
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Reported-by: Xi Ruoyao <xry111@mengyan1223.wang>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Xi Ruoyao <xry111@mengyan1223.wang>
Acked-by: Kees Cook <keescook@chromium.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/alpine.DEB.2.21.1907102140340.1758@nanos.tec.linutronix.de