Look for build-id-based separate debug files under the sysroot

Look for build-id-based separate debug files under the sysroot

When looking for a separate debug file that matches a given build-id,
GDB only looks in the host's debug dir (typically /usr/lib/debug).  This
patch makes it look in the sysroot as well.  This is to match the
behavior of GDB when using debuglink-based separate debug files,
introduced in :

402d2bfec42 ("Look for separate debug files in debug directories under a sysroot.")

In the following example, my sysroot is "/tmp/sysroot" and I am trying
to load symbols for
/tmp/sysroot/usr/lib/arm-linux-gnueabihf/gconv/EBCDIC-AT-DE.so.  This is
the current behavior:

    (gdb) file /tmp/sysroot/usr/lib/arm-linux-gnueabihf/gconv/EBCDIC-AT-DE.so
    Reading symbols from /tmp/sysroot/usr/lib/arm-linux-gnueabihf/gconv/EBCDIC-AT-DE.so...

    Looking for separate debug info (build-id) for /tmp/sysroot/usr/lib/arm-linux-gnueabihf/gconv/EBCDIC-AT-DE.so
      Trying /usr/lib/debug/.build-id/f3/d6594d2600e985812cd4ba2ad083ac2aceae22.debug... no, unable to compute real path

    <snip>
    (No debugging symbols found in /tmp/sysroot/usr/lib/arm-linux-gnueabihf/gconv/EBCDIC-AT-DE.so)

With this patch:

    (gdb) file /tmp/sysroot/usr/lib/arm-linux-gnueabihf/gconv/EBCDIC-AT-DE.so
    Reading symbols from /tmp/sysroot/usr/lib/arm-linux-gnueabihf/gconv/EBCDIC-AT-DE.so...

    Looking for separate debug info (build-id) for /tmp/sysroot/usr/lib/arm-linux-gnueabihf/gconv/EBCDIC-AT-DE.so
      Trying /usr/lib/debug/.build-id/f3/d6594d2600e985812cd4ba2ad083ac2aceae22.debug... no, unable to compute real path
      Trying /tmp/sysroot/usr/lib/debug/.build-id/f3/d6594d2600e985812cd4ba2ad083ac2aceae22.debug... yes!
    Reading symbols from /tmp/sysroot/usr/lib/debug/.build-id/f3/d6594d2600e985812cd4ba2ad083ac2aceae22.debug...

In the original code, there is a suspicious "abfd.release ()" in
build_id_to_debug_bfd, that I don't understand.  If a file with the
right name exists but its build-id note doesn't match, we release (leak)
our reference, meaning the file will stay open?  I removed it in the new
code, so that the reference is dropped if we end up not using that file.
I tested briefly by corrupting a separate debug file to trigger this
code, nothing exploded.

gdb/ChangeLog:

	* build-id.c (build_id_to_debug_bfd_1): New function.
	(build_id_to_debug_bfd): Look for separate debug file in
	sysroot.