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

This reverts commit 34e1f405dc7386a04007a55cdd3efa848c026ead.
Provide <sys/user.h>.

strace wants this, for example.

Change-Id: Icb4d64bde392abbc091fa3375a4fa19b2707b12c
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
libc: remove std=gnu99 and fix some warnings

Signed-off-by: Domenico Marino <johnlemon.lol@gmail.com>
libm: fix some warnings

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