x86: user put_user_x instead of all variants.

Follow the pattern, and define a single put_user_x, instead
of defining macros for all available sizes. Exception is
put_user_8, since the "A" constraint does not give us enough
power to specify which register (a or d) to use in the 32-bit
common case.

Signed-off-by: Glauber Costa <gcosta@redhat.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>