toolchain: have check_glibc to search deeper for ld-linux

ld-linux*.so may not be present in lib/ directory, it could be
in lib32 and/or lib64 only. But check_glibc reports
"Incorrect selection of the C library" in this case, which is
not true.
Fixed by extending the search to  SYSROOT/*/*.

Signed-off-by: Jean-Mickael Guerin <jean-mickael.guerin@6wind.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>