[gdb/testsuite] Fix gdb.base/jit-elf-so.exp for remote target

[gdb/testsuite] Fix gdb.base/jit-elf-so.exp for remote target

With test-case gdb.base/jit-elf-so.exp and target board
remote-gdbserver-on-localhost (using REMOTE_TARGET_USERNAME) we run into some
failures.

Fix these by:
- setting jit_libname with the name as returned by gdb_load_shlib
- allowing the libraries to be prefixed with the remote target directory.

Tested on x86_64-linux.

Co-Authored-by: Ivan Tetyushkin <ivan.tetyushkin@syntacore.com>