toolchain-external: improve lib subdirectory matching

The toolchain from the Cavium Networks Octeon SDK provides a sysroot
with library directories lib32, lib32-fp, lib64 and lib64-fp. The -fp
variants are used for processors with hardware floating point unit, such
as the Octeon III variants.

When specifying -march=octeon3 in BR2_TARGET_OPTIMIZATION, the toolchain
will use lib32-fp, but currently Buildroot does not accept that pattern.

This patch improves the matching by accepting lib(32|64)?([^/]*)? as lib
directory name.

Signed-off-by: Bai Yingjie <byj.tea@gmail.com>
[ThomasDS: update commit message]
Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
[Thomas: add comment above the function being modified to illustrate
the various cases we try to handle.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>