Exec gdb and lldb in rust-* wrappers

Exec gdb and lldb in rust-* wrappers

This way the process we get by calling rust-{gdb,lldb} is an actual
{gdb,lldb} instance and not (perhaps surprisingly) a script waiting for
the debugger process to finish. Thus, sending a SIGINT to the spawned
process stops execution of the child, for example.