Add gnu_dev_* symbols

There are three Linux specific symbols gnu_dev_major(), gnu_dev_minor()
 and gnu_dev_makedev() that were not implemented in musl libc.
This patch adds these by reusing major(), minor() and makedev() code
from musl macros.
It also uses the macros from newer version of musl, that work around
some bugs, see musl changelog
https://github.com/rofl0r/musl/commit/9b732fe51b814468a08c84d13238b4c83a42451a

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Constantine Peresypkin <constantine.peresypk@rackspace.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>