[XTENSA] Add support for executable/non-executable feature in the mmu

Newer processor versions starting with Xtensa6/LX2 support an 'executable'
bit for memory pages. This bit replaces the 'valid' bit, so it must be
always set to one for older processor versions. To mark a page invalid, we now
set the cache-attributes to b11, which is backward compatible.

Signed-off-by: Chris Zankel <chris@zankel.net>