gdb/testsuite/dwarf: add support to generate DWARF 5 split compile units

gdb/testsuite/dwarf: add support to generate DWARF 5 split compile units

Add support to the DWARF assembler to generate DWARF 5 split compile
units.  The assembler knows how to generate DWARF < 5 split compile
units (fission), DWARF 5 compile units, but not DWARF 5 split compile
units.  What's missing is:

 - using the right unit type in the header: skeleton for the unit in the
   main file and split_compile for the unit in the DWO file
 - have a way for the caller to specify the DWO ID that will end up in
   the unit header

Add a dwo_id parameter to the cu proc.  In addition to specifying the
DWO ID, the presence of this parameter tells the assembler to use the
skeleton or split_compile unit type.

This is used in a subsequent patch.

Change-Id: I05d9b189a0843ea6c2771b1d5e5a91762426dea9
Approved-By: Tom Tromey <tom@tromey.com>