rust-gdb on OS X

On OS X, I noticed that the current implementation of multirust does not seem to copy the `rust-gdb` script (https://github.com/rust-lang/rust/blob/master/src/etc/rust-gdb) into the `~/.multirust/toolchains/stable/bin/` directory. For other toolchains, like nightly, etc. I have not tested, but I assume the same behavior. Only the `rust-lldb` pretty-printer script seems to be copied.

I personally installed `gdb` on my MacBook because I like it more than `rust-lldb`, so I would appreciate it if the `rust-gdb` script could be copied into `~/.multirust/toolchains/stable/bin/` as well.

After all, the helper script `rust-gdb` which is in my `PATH` (i.e., `/usr/local/bin/rust-gdb`) actually gets installed, but fails on the last line

``` bash
exec "$sysroot/bin/$rust_cmd" $extra_flags "$@"
```

because - as stated above - cannot find `~/.multirust/toolchains/stable/bin/rust-gdb`.

**Edit:** Maybe that is actually a flaw of the actual rust installation process, but so far I haven't used rust, only multirust.