binutils: install libiberty

Some packages (actually, just oprofile) need to link against libiberty.
This option just installs libiberty.a so it has no effect on the target,
therefore it's not needed to add a config option for it.

Before binutils-2.24, there was a bug in libiberty/Makefile.in that
caused libiberty to be installed regardless of the
--enable-install-libiberty option. This problem wasn't noticed before
because binutils-2.24 is not selected on any of the autobuilders: the
version can only be selected if an internal toolchain is used, and it
defaults to 2.21.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>