Improve external toolchain logic to support IA32 Sourcery CodeBench toolchain

Improve external toolchain logic to support IA32 Sourcery CodeBench toolchain

The IA32 Sourcery CodeBench toolchain has a relatively special
structure, with the following multilib variants:

 * Intel Pentium 4, 32 bits, the multilib variant is in ./ relative to
   the main sysroot, with the libraries in the lib/ directory.

 * Intel Xeon Nocona, 64 bits, the multilib variant is in ./ relative
   to the main sysroot, with the libraries in the lib64/ directory.

 * Intel Atom 32 bits, the multilib variant is in atom/ relative to
   the main sysroot, with the libraries in the lib/ directory.

 * Intel Core 2 64 bits, the multilib variant is in core2/ relative to
   the main sysroot, with the libraries in lib64/ directory.

So the first two variants are in the same sysroot, only the name of
the directory for the libraries is different.

Therefore, we introduce a new ARCH_LIB_DIR variable, which contains
either 'lib' or 'lib64'. This variable is defined according to the
location of the libc.a file for the selected multilib variant, and is
then used when copying the libraries to the target and to the staging
directory.

In addition to this, we no longer use the -print-multi-directory to
get the ARCH_SUBDIR, since in the case of the 64 bits variants of this
toolchain, it returns just '64' and not a real path. Instead, we
simply compute the difference between the arch-specific sysroot and
the main sysroot.

We also take that opportunity to expand the documentation on the
meaning of the different variables.

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