[SPARC64]: Fix _PAGE_EXEC handling.

First of all, use the known _PAGE_EXEC_{4U,4V} value instead
of loading _PAGE_EXEC from memory.  We either know which one
to use by context, or we can code patch the test.

Next, we need to check executability of a PTE in the generic
TSB miss handler.

Signed-off-by: David S. Miller <davem@davemloft.net>