random: Use getentropy on macOS when available.
* random/rndlinux.c [__APPLE__ && __MACH__] (getentropy): Declare. (_gcry_rndlinux_gather_random): Check the symbol and use getentropy.
- GnuPG-bug-id: T5268
- Suggested-by: David Carlier
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>