external-toolchain: add support for recent Linaro toolchains

Starting from 2012.03, the Linaro toolchains have separated the GCC
support libraries (libstdc++, libgcc_s) from the sysroot itself. So we
no longer have the case where all libraries are inside the sysroot, as
we had for all the previously supported toolchains.

Therefore, we add some logic to detect if such a separate directory is
used for GCC support libraries, and if it's the case, we make sure
that we take into account this directory when creating our own
sysroot, and when copying libraries to the target filesystem.

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