toolchain-external: support toolchains with lib32 directories

Some toolchains, like the Cavium Networks' one, have lib32/ and lib64/
directories, while the standard lib/ is empty. To find libc.a, buildroot
currently only looks in lib/ and lib64/. This patch extends the search
to lib32/ as well.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>