x86: make GDT per-CPU

x86: make GDT per-CPU

The major issue with supporting a significantly larger number of
physical CPUs appears to be the use of per-CPU GDT entries - at
present, x86-64 could support only up to 126 CPUs (with code changes
to also use the top-most GDT page, that would be 254). Instead of
trying to go with incremental steps here, by converting the GDT itself
to be per-CPU, limitations in that respect go away entirely.

Signed-off-by: Jan Beulich <jbeulich@novell.com>