x86: Add sysctl flag CTLFLAG_TUN to loader tunables

The following sysctl variables are actually loader tunables. Add sysctl
flag CTLFLAG_TUN to them so that sysctl -T will report them correctly.

1. machdep.idle
2. machdep.idle_apl31

No functional change intended.

Reviewed by: kib, imp
Approved by: re (gjb)
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D42113

(cherry picked from commit 149b9c234b00ff902718e1f76c809609a5465198)
(cherry picked from commit 9eac775b415bf54080a98a9cf62524b8a6817595)