LINUX: Avoid building rand-fortuna-kernel.o

LINUX: Avoid building rand-fortuna-kernel.o

Currently, we build rand-fortuna-kernel.o for libafs on all platforms,
even though we only use the fortuna RNG on AIX, DragonFlyBSD, HP-UX,
and Irix. Everywhere else, our RAND_bytes() in
src/crypto/hcrypto/kernel/rand.c uses osi_readRandom() instead of
going through heimdal.

Building rand-fortuna.c causes occasional build headaches for the
kernel on Linux (see cc7f942, "LINUX: Disable kernel fortuna large
frame errors"). The most recent instance of this is that Linux 5.6
removes the definition for struct timeval, which is referenced in
rand-fortuna.c.

The Linux kernel is constantly changing, and so trying to keep
rand-fortuna.c building on Linux seems like a waste of ongoing effort.
So, just stop building rand-fortuna-kernel.o on Linux. The original
intent of building this file on all platforms was to avoid bitrot, so
still keep building rand-fortuna-kernel.o on all other platforms even
when it's not used; just avoid it on Linux specifically, the platform
that requires the most effort.

To accomplish this, move rand-fortuna-kernel.o from AFSAOBJS to
AFS_OS_OBJS, and remove it from the Linux-only AFSPAGOBJS.

[1.8.x: The 1.8 branch does not contain the commits that introduced
-Wno-error=frame-larger-than= (cc7f942a and 54150f38), so we can skip
removing the references to -Wno-error=frame-larger-than=.]

Reviewed-on: https://gerrit.openafs.org/14084
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit b8088b49dec23da19406fcb014e7100695dc8322)

Change-Id: Iad0d1af5ffd79c576ddbc253b0037b9772187350
Reviewed-on: https://gerrit.openafs.org/14094
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Tested-by: Cheyenne Wills <cwills@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>