toolchain-external: fix support of uClibc-ng toolchains

The uClibc-ng dynamic loader is called ld-uClibc.so.1, but gcc is not
patched specifically for uClibc-ng, so it continues to generate
binaries that expect the dynamic loader to be named ld-uClibc.so.0,
like with the original uClibc.

Therefore, when a uClibc-ng toolchain is used as an external
toolchain, we need to create an additional symbolic link to make
uClibc-ng systems work properly.

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