tile: fix personality bits handling upon exec()

Historically, the top three bytes of personality have been used for
things such as ADDR_NO_RANDOMIZE, which made sense only for specific
architectures.

We now however have a flag there that is general no matter the
architecture (UNAME26); generally we have to be careful to preserve the
personality flags across exec().

This patch fixes tile architecture not to forcefully overwrite
personality flags during exec().

In addition to that, we fix two other things along the way:

- exec_domain switching is fixed -- set_personality() should always
  be used instead of directly assigning to current->personality.
- as pointed out by Arnd Bergmann, PER_LINUX_32BIT is not used anywhere
  by tile, so let's just drop that in favor of PER_LINUX

Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Acked-by: Chris Metcalf <cmetcalf@tilera.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>