toolchain: fix installing gconv libs with multi-arch toolchain

For a multi-arch toolchain, gconv modules are in a sub-directory named
after the machine gcc targets. This is the case, for example, for the
Linaro ARM 2014.09 toolchain, which has the gconv modules in (relative
to the sysroot):
    /usr/lib/arm-linux-gnueabihf/gconv

while the Sourcery CodeBench ARM 2014.05 (non-multi-arch) has them in:
    /usr/lib/gconv

So, to catter for both cases, search both paths. We want to favour the
machine-specific gconv modules over potentially existing "generic" ones,
so we first search that (if it exists) and fallback to looking in the
generic location.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>