hurd: Fix __writev_nocancel_nostatus

* sysdeps/mach/hurd/Makefile [subdir=misc] (sysdep_routines): Add
writev_nocancel writev_nocancel_nostatus.
* sysdeps/mach/hurd/not-cancel.h (__writev_nocancel_nostatus): Replace
macro with function declaration (with hidden prototype in libc).
(__writev_nocancel): New function declaration (with hidden prototype in libc).
* sysdeps/mach/hurd/writev_nocancel_nostatus.c: New file.
* sysdeps/posix/writev_nocancel.c: New file, includes writev.c to make a
nocancel variant that calls __write_nocancel.
* sysdeps/posix/writev.c (writev): Do not define alias if __writev is
renamed.