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.