x86, mm: Redesign get_user with a __builtin_choose_expr hack

Instead of using a bitfield, use an odd little trick using typeof,
__builtin_choose_expr, and sizeof.  __builtin_choose_expr is
explicitly defined to not convert its type (its argument is required
to be a constant expression) so this should be well-defined.

The code is still not 100% preturbation-free versus the baseline
before 64-bit get_user(), but the differences seem to be very small,
mostly related to padding and to gcc deciding when to spill registers.

Cc: Jamie Lokier <[email protected]>
Cc: Ville Syrjälä <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Russell King <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: H. J. Lu <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: H. Peter Anvin <[email protected]>

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