external-toolchain: recognize uClibc 64 bits toolchains

With uClibc 64 bits toolchain, the dynamic loader is named
ld64-uClibc.so.0 and not ld-uClibc.so.0. So, this commit adjust the
uClibc detection code for external toolchains.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Yann E. MORIN <yann.morin.1998@anciens.enib.fr>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>