linux-user: Support futex_time64

Add support for host and target futex_time64. If futex_time64 exists on
the host we try that first before falling back to the standard futex
syscall.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <d9390e368a9a1fd32d52aa771815e6e3d40cb1d4.1584571250.git.alistair.francis@wdc.com>
[lv: define sys_futex() if __NR_futex is defined (fix bug on 32bit host),
     remove duplicate get_errno()]
Signed-off-by: Laurent Vivier <laurent@vivier.eu>