ext-toolchains: take into account other Glibc dynamic loader variants

External toolchains using Glibc have different names for the dynamic
loader. Some of them name it ld-linux.so.*, while some others (such as
the PowerPC and MIPS CodeSourcery toolchains) name it simply ld.so.*.

Therefore, we fix the glibc detection code to handle this case.

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>