Remove target: prefix from gdb_sysroot in find_separate_debug_file

Remove target: prefix from gdb_sysroot in find_separate_debug_file

I noticed that, when using gdbserver, gdb might print:

Reading /usr/lib/debug/lib64//libcap.so.2.48-2.48-4.fc36.x86_64.debug from remote target...
Reading target:/usr/lib/debug/lib64//libcap.so.2.48-2.48-4.fc36.x86_64.debug from remote target...

The second line has the "target:" prefix, but from the code it's clear
that this string is being passed verbatim to gdbserver -- which seems
wrong.

I filed PR remote/29929 for this.

The problem here is that find_separate_debug_file uses gdb_sysroot
without checking to see if it starts with the "target:" prefix.  This
patch changes this code to be a little more careful.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29929