tile: avoid a "label not used" warning in do_page_fault()

There are two different ifdef cases where the label is used,
but if neither is true, the label is unused and the compiler
generates a warning.

Refactor the code the way x86 does so that there is a
do_page_fault() that just does exception handling for
context tracking, and make __do_page_fault() a static inline
so that various cases can just return instead of doing a
jump to "done".

Signed-off-by: Chris Metcalf <cmetcalf@ezchip.com>