toolchain-external: cover multilib toolchains with lib/<variant> layout

The toolchain from the Cavium Octeon SDK has a sysroot layout as follows:

./lib32
./lib32/octeon2
./lib32-fp
./lib64
./lib64/octeon2
./lib64-fp
./usr
./usr/lib
./usr/lib32
./usr/lib32/octeon2
./usr/lib32-fp
./usr/lib64
./usr/lib64/octeon2
./usr/lib64-fp
./usr/bin
./usr/bin32
./usr/bin32-fp
./usr/bin64-fp
./usr/libexec
./usr/libexec32
./usr/libexec32-fp
./usr/libexec64-fp
./usr/sbin
./usr/sbin32
./usr/sbin32-fp
./usr/sbin64-fp
./usr/include
./usr/share
./sbin
./sbin32
./sbin32-fp
./sbin64-fp
./etc
./var

with the following selections:
- lib64          : default
- lib64/octeon2  : -march=octeon2
- lib64-fp       : -march=octeon3
- lib32          : -mabi=n32
- lib32/octeon2  : -mabi=n32 -march=octeon2
- lib32-fp       : -mabi=n32 -march=octeon3

In case of '-mabi=n32 -march=octeon2' (but same is true for n64+octeon2)the
original Buildroot toolchain logic would copy both the libraries in
lib32 as the subdirectory lib32/octeon2, which means that every library is
installed twice (but only one of each is really needed).

While ARCH_LIB_DIR is determined by the location of libc.a, which in this
case is effectively:
    <sysroot>/usr/lib32/octeon2/libc.a
the variable only retains 'lib32' and not 'lib32/octeon2' as expected.

To make Buildroot cope with this style of toolchain layout, we need to adapt
the calculation of ARCH_LIB_DIR to also include the second part.
This, in turn, means that ARCH_LIB_DIR is no longer guaranteed to be a
singular path component, resulting in some additional changes.

Certain older Linaro toolchains actually had the same layout. Libraries were
located in lib/<tuple> rather than lib directly. Previously, this was
handled by adding a toolchain-specific fixup that creates a symlink
lib/<tuple> -> lib, but with this patch this would no longer be needed.
Note that one difference with the Octeon case is that these Linaro
toolchains are not actually multilib, i.e. there is just one location with
the libraries and thus there is no problem with duplicated libraries.

Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

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