Fix PR gdb/17917 Lookup build-id in remote binaries

Fix PR gdb/17917 Lookup build-id in remote binaries

GDB doesn't support loading debug files using build-id from remote
target filesystems.

This is the case when gdbserver attached to a process and a gdb target
remote occurs over tcp.

With this change we make build-id lookups possible:

    (gdb) show debug-file-directory
    The directory where separate debug symbols are searched for is "/usr/local/lib/debug".
    (gdb) set debug-file-directory /usr/lib/debug
    (gdb) show sysroot
    The current system root is "target:".
    (gdb) target extended-remote :46615
    Remote debugging using :46615
    warning: Can not parse XML target description; XML support was disabled at compile time
    Reading /usr/sbin/mariadbd from remote target...
    warning: File transfers from remote targets can be slow. Use "set sysroot" to access files locally instead.
    Reading /usr/sbin/mariadbd from remote target...
    Reading symbols from target:/usr/sbin/mariadbd...
    Reading /usr/lib/debug/.build-id/6e/0a874dca5a7ff831396ddc0785d939a192efe3.debug from remote target...
    Reading /usr/lib/debug/.build-id/6e/0a874dca5a7ff831396ddc0785d939a192efe3.debug from remote target...
    Reading symbols from target:/usr/lib/debug/.build-id/6e/0a874dca5a7ff831396ddc0785d939a192efe3.debug...
    Reading /lib/x86_64-linux-gnu/libpcre2-8.so.0 from remote target...
    ...

Before this change, the lookups would have been (GNU gdb (GDB) Fedora 10.2-3.fc34):

    (gdb) target extended-remote :46615
    Remote debugging using :46615
    Reading /usr/sbin/mariadbd from remote target...
    warning: File transfers from remote targets can be slow. Use "set sysroot" to access files locally instead.
    Reading /usr/sbin/mariadbd from remote target...
    Reading symbols from target:/usr/sbin/mariadbd...
    Reading /usr/sbin/0a874dca5a7ff831396ddc0785d939a192efe3.debug from remote target...
    Reading /usr/sbin/.debug/0a874dca5a7ff831396ddc0785d939a192efe3.debug from remote target...
    Reading /usr/lib/debug//usr/sbin/0a874dca5a7ff831396ddc0785d939a192efe3.debug from remote target...
    Reading /usr/lib/debug/usr/sbin//0a874dca5a7ff831396ddc0785d939a192efe3.debug from remote target...
    Reading target:/usr/lib/debug/usr/sbin//0a874dca5a7ff831396ddc0785d939a192efe3.debug from remote target...
    Missing separate debuginfo for target:/usr/sbin/mariadbd
    Try: dnf --enablerepo='*debug*' install /usr/lib/debug/.build-id/6e/0a874dca5a7ff831396ddc0785d939a192efe3.debug
    (No debugging symbols found in target:/usr/sbin/mariadbd)

Observe it didn't look for
/usr/lib/debug/.build-id/6e/0a874dca5a7ff831396ddc0785d939a192efe3.debug
on the remote target (where it is) and expected them to be installed
locally.

As a minor optimization, this also changes the build-id lookup such that
if sysroot is empty, no second lookup of the same location is performed.

Change-Id: I5181696d271c325a25a0805a8defb8ab7f9b3f55
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=17917