Move arm user.h definitions to sys/user.h.

There is no uapi user.h file for arm, it was included by accident.
Move the user struct definition into the file to follow the pattern
used by the other architectures.

Change-Id: Ib9cea0deca551c9268382ddd6de9202fd32ef941

Conflicts:

	libc/include/sys/user.h

Signed-off-by: Domenico Marino <johnlemon.lol@gmail.com>