Fix sources needed for lld.

lld always needs these DWARF sources, as well as other default and extra
tools. XDL seems to be the best fit list.

Remove MK_LLD_IS_LD check from SRCS_MIW which is now reduced to just a
few files for llvm-objdump.

Sponsored by: Dell EMC
Differential Revision: https://reviews.freebsd.org/D15915