binutils: default to 2.24

2.22 is getting quite old (Nov 2011) and we've recently added 2.25. 2.24 has
been used for the "new" architectures for a while and there's no known
issues going to 2.24 for the rest, so bump the default version.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>