[binutils] Handle DW_UT_skeleton/split_compile in process_debug_info

[binutils] Handle DW_UT_skeleton/split_compile in process_debug_info

With this exec:
...
$ gcc -gsplit-dwarf hello.c -gdwarf-5
...
we run into:
...
$ readelf -w a.out > READELF
readelf: Warning: CU at offset c7 contains corrupt or unsupported unit type: 4.
readelf: Warning: CU at offset c7 contains corrupt or unsupported unit type: 4.
...

Fix this by handling DW_UT_skeleton and DW_UT_split_compile in
process_debug_info.

Note that this just adds the parsing of DWO_id, but not yet any printing of
it.

Tested on x86_64-linux.

binutils/ChangeLog:

2021-02-09  Tom de Vries  <tdevries@suse.de>

	PR binutils/27386
	* dwarf.c (process_debug_info): Handling DW_UT_skeleton and
	DW_UT_split_compile.