arch/x86_64: Use modern CR0 assembly

The 16 bit bootstrap code for SMP CPUs was using the 286-era "lmsw"
instruction (load machine status word) to set the protected bit in CR0
(which is the modern evolution of the same register), presumably
because this is 16 bit code and we can't move a dword into CR0.

But that's wrong, because the full instruction set *is* available in
real mode on a 386, you just have to use a operand size prefix to get
to it, which the assembler emits for you automatically when you use
the .code16 directive.

Write this conventionally and use modern (e.g. 1986-era) instructions.
It also has the advantage of not confusing much more modern
hypervisors like ACRN by issuing instructions they (and I!) never knew
existed.

Fixes #35076

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>