toolchain-external: fix bug #5054

The external toolchain logic checks (and finds) the proper ARCH_LIB_DIR
and forcibly copies it to */lib even if it's in */lib64
This is all well until the check is done for create_lib64_symlinks which
only verifies if ARCH_SYSROOT_DIR/lib64 is a symlink, which in some
toolchain it's a real directory (like sourcery x86_64 2012.09) and thus
doesn't make the symlink in the target.

Fix this by also checking for a real directory.

Easily reproducible by running "make qemu_x86_64_defconfig", switching
to an external toolchain before build, building and then trying to run
the resulting image.

Closes bug #5054

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>