gdb: fix a target: prefix issue in find_separate_debug_file

Following on from the previous commit, this commit fixes the two KFAIL
in gdb.base/sysroot-debug-lookup.exp when not using the
native-extended-gdbserver board.

The failures in this test, when using the 'unix' board, are logged as
bug PR gdb/31804.  The problem appears to be caused by the use of the
child_path function in find_separate_debug_file.

What happens on the 'unix' board is that the file is specified to GDB
with a target: prefix, however GDB spots that the target filesystem is
local to GDB and so opens the file without a target: prefix.  When we
call into find_separate_debug_file the DIR and CANON_DIR arguments,
which are computed from the objfile_name() no longer have a target:
prefix.

However, in this test if the file was opened with a target: prefix,
then the sysroot also has a target: prefix.  When child_path is called
it looks for a common prefix between CANON_DIR (from the objfile_name)
and the sysroot.  However, the sysroot still has the target: prefix,
which means the child_path() call fails and returns nullptr.

What I think we need to do is this: if the sysroot has a target:
prefix, and the target filesystem is local to GDB, then we should
strip the target: prefix from the sysroot, just as we do when opening
a file (see gdb_bfd_open in gdb_bfd.c).

Now, when we make the child_path() call neither the sysroot nor
CANON_DIR will have a target: prefix, the child_path() call will
succeed, and GDB will correctly find the debug information.

There is just one remaining issue, the last path we look in when
searching for debug information is built by starting with the sysroot,
then adding the debug directory, see this line:

  debugfile = path_join (target_prefix_str, root.c_str (),
                         debugdir.get (), base_path, debuglink);

The target_prefix_str is set to target: if DIR has a target: prefix,
and DIR should have a target: prefix if the file we're looking for was
opened with a target: prefix.  However, in our case the file was in a
local filesystem so GDB stripped the prefix off.

The sysroot however, does have the target: prefix, and the test is
expecting to see GDB look within a file with the target: prefix.

Given that the above line is about looking within a sub-directory of
the sysroot, I think we should not be stripping the target: prefix off
the sysroot path (as we do when building ROOT), instead, we should, in
this case, not use TARGET_PREFIX_STR, and instead just use GDB's
sysroot as it is (in this case with the target: prefix).

With these fixes in place I now see no failures when using the 'unix',
'native-gdbserver', or 'native-extended-gdbserver' boards with this
test, and the KFAILs can be removed.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31804