external-toolchain: support 64 bits glibc toolchains

On 64 bits glibc toolchains, the dynamic loader is named
ld-linux-x86-64.so and not simply ld-linux.so. So, adjust the
detection of the C library accordingly.

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>