x86: set strong uncacheable where UC is really desired

Also use _PAGE_PWT for all the mappings which need uncache mapping.
Instead of existing PAT2 which is UC- (and can be overwritten by MTRRs),
we now use PAT3 which is strong uncacheable.

This makes it consistent with pgprot_noncached()

Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>