toolchain-external: fix uClibc-ng 64bit dynamic loader link

Commit 34f95bf9 (toolchain-external: fix support of uClibc-ng toolchains,
2015-07-13) added the missing ld-uClibc.so.1 dynamic linker symlink that
binaries expect when linked with uClibc-ng. However on 64bit targets the
linker is called ld64-uClibc.so.1. Handle that case as well.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>