MFC r263749,267146:

>r267146 | imp | 2014-06-05 22:08:55 -0600 (Thu, 05 Jun 2014) | 4 lines
>Restore comments accidentally removed.

>r263749 | imp | 2014-03-25 16:08:31 -0600 (Tue, 25 Mar 2014) | 18 lines
>Rather than require a makeoptions DEBUG to get debug correct,
>add it in kern.mk, but only if we're using clang. While this
>option is supported by both clang and gcc, in the future there
>may be changes to clang which change the defaults that require
>a tweak to build our kernel such that other tools in our tree
>will work. Set a good example by forcing -gdwarf-2 only for
>clang builds, and only if the user hasn't specified another
>dwarf level already. Update UPDATING to reflect the changed
>state of affairs. This also keeps us from having to update
>all the ARM kernels to add this, and also keeps us from
>in the future having to update all the MIPS kernels and is
>one less place the user will have to know to do something
>special for clang and one less thing developers will need
>to do when moving an architecture to clang.