DWARF2 fixes for LLVM

Default to DWARF2 sections, since that is the only format supported by
our ancient binutils.  While here, pull in r195391, r198385 and r198389
from LLVM trunk, which fixes issues with "-gdwarf-2".

The patches were initially imported from FreeBSD to our ports tree, and
are now being brought to src/contrib, once the switch to in-tree LLVM is
complete.

OK patrick@
add patch for e4b38b61