Remove DIAGNOSTIC from the kernel config of low-end arm systems.  Sanity
checks such as vmem_check() can make a low-end system go completely
unresponsive for as much as 3 seconds out of every 10.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271050 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f