Create <tuple>/lib -> <sysroot>/lib symlink before installing cross gcc

This commit solves bug #1051. The problem in this bug in that WebKit
compiles a sample C program, which uses WebKit. As WebKit is written
in C++, even though the program it built with CROSS-gcc, it must be
linked with libstdc++. However, CROSS-gcc can't find the libstdc++ has
it's hidden inside <sysroot>/<tuple>/lib.

Therefore, this commit creates a symbolic link <sysroot>/<tuple>/lib
-> <sysroot>/lib before running the CROSS-gcc installation. While this
may look like a hack, this is the solution used by both Crosstool-NG
and OpenWRT.

Moreover, with this symbolic link in place, I think bug #1741 may also
be solved. The problem in this bug is that the linker tries to link
against /lib/libc.so.0. This is due to the fact that the linker finds
a libc.so script file in the original toolchain location and not
inside the copy of the toolchain sysroot in $(STAGING_DIR). As the
script file is found outside of the current toolchain sysroot, ld
considers the script has non-sysrooted, and therefore doesn't prefix
all paths found in the script file (such as /lib/libc.so.0) with the
sysroot path, leading to the failure.

So, in details, this commit :

 * Adds a BR2_ARCH_IS_64 invisible config knob that is used to know if
   the arch is a 64 bits architecture or not.

 * Creates the <sysroot>/<tuple>/lib -> <sysroot>/lib symbolic link,
   and the <sysroot>/<tuple>/lib64 -> <sysroot>/lib64 symbolic link if
   needed.

 * Fixes the external toolchain sysroot detection code so that the
   'sed' replacement is done *after* the readlink -f evaluation.

I have tested this by building ARM, x86 and x86_64 toolchains with
Buildroot, and then use these toolchains as external toolchains to
build a full X.org/Gtk/WebKit/Midori stack. I have also done a
complete ARM Buildroot internal toolchain build with the same full
X.org/Gtk/WebKit/Midori stack.

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

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