[binutils] Handle absolute DW_AT_dwo_name

[binutils] Handle absolute DW_AT_dwo_name

With an exec:
...
$ pwd
/home/vries/tmp
$ gcc /home/vries/tmp/src/hello.c -gsplit-dwarf -c \
  -o /home/vries/tmp/obj/hello.o
...
I get:
...
$ readelf -w obj/hello.o > READELF
readelf: Warning: Unable to load dwo file: \
  /home/vries/tmp//home/vries/tmp/obj/hello.dwo
...

The dwo file name is listed here:
...
    <20>   DW_AT_GNU_dwo_name: /home/vries/tmp/obj/hello.dwo
    <24>   DW_AT_comp_dir    : /home/vries/tmp
...

The standard states about the DW_AT_dwo_name attribute:
...
value is a null-terminated string containing the full or relative path name
(relative to the value of the DW_AT_comp_dir attribute, see below) of the
object file that contains the full compilation unit.
...

So, readelf shouldn't try to prefix an absolute path with DW_AT_comp_dir.

Fix this in load_dwo_file by handling the absolute path case.

binutils/ChangeLog:

2021-02-10  Tom de Vries  <tdevries@suse.de>

	PR binutils/27391
	* dwarf.c (load_dwo_file): Handle case that name is absolute path.