MFC r260880 (by kaiw, from projects/elftoolchain):

  * Make die_mem_offset() be able to handle DW_AT_data_member_location
    attributes generated by Clang 3.4.
  * Document how different compilers generate DW_AT_data_member_location
    attributes differently.
  * Document the quirks about DW_FORM_data[48].

This is a slightly modified version, adapted to work with the old
libdwarf in stable/9 and stable/10.  It should fix DTrace on these
branches, when the kernel is compiled with clang 3.4.

Note that you have to build *and* install the CTF tools first, before
building the kernel.  Otherwise you can possibly still get error
messages similar to "failed to copy type of 'pr_uid': Type information
is in parent and unavailable", when attempting to run dtrace(1).

Submitted by:	kaiw