x86/boot: Make alternative patching NMI-safe

x86/boot: Make alternative patching NMI-safe

During patching, there is a very slim risk that an NMI or MCE interrupt in the
middle of altering the code in the NMI/MCE paths, in which case bad things
will happen.

The NMI risk can be eliminated by running the patching loop in NMI context, at
which point the CPU will defer further NMIs until patching is complete.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>