hurd: Fix local PLT

* include/sys/random.h (__getrandom): Add hidden prototype.
* stdlib/getrandom.c (getrandom): Rename to hidden definition __getrandom.
Add weak alias.
* sysdeps/mach/hurd/getrandom.c (getrandom): Likewise.
* sysdeps/unix/sysv/linux/getrandom.c (getrandom): Likewise.
* sysdeps/mach/hurd/getentropy.c (getentropy): Use __getrandom instead of
getrandom.