I've written and tested one.
If anyone's interested, you can get it here:
http://pastebin.com/KGEjxCV2
Example:
Will download, extract, build and install binutils and gcc targeting arm-none-eabi. There are options to download without building, build from existing archives, build one or the other instead of both, change the installation path (defaults to $HOME/opt/cross) and the versions (which default to 2.24 for binutils and 4.9.2 for gcc), and set configure options and makefile targets.
[edit] It doesn't fetch dependencies so you will need to install GMP etc. separately.
Adding that code shouldn't be difficult though.