x86: merge __put_user_asm and its user.

Move both __put_user_asm and __put_user_size to
uaccess.h. i386 already had a special function for 64-bit access,
so for x86_64, we just define a macro with the same name.
Note that for X86_64, CONFIG_X86_WP_WORKS_OK will always
be defined, so the #else part will never be even compiled in.

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>