* sysdeps/unix/sysv/linux/x86_64/pthread_once.S: Fix many
leftovers from the ia32 code.
* sysdeps/unix/sysv/linux/i386/pthread_once.S: Remove unneccessary
memory load.
(clear_once_control): Don't load %esi.
* sysdeps/x86_64/tls.h: Remove all traces of segment descriptor
handling.
* sysdeps/unix/sysv/linux/x86_64/fork.c: New file.
