linux: improve x86 page table handling performance

linux: improve x86 page table handling performance

Where possible,
- use hypercalls instead of writing to read-only pages
- fold TLB flushes into page table update hypercalls
- on PAE, use single-access updates instead of two-access ones

The single change to PAE pte_clear() yields a 25-30% boost for kernel
builds on a 4x2x2 CPUs, 8Gb box; the other changes together yield
improvements of 2-5%.

Also, adjust backward compatibility handling in a few more places.

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