Makefile: fix stripping of ld.so and libpthread for merged /usr

If 'lib' is a symlink (as is the case when BR2_ROOTFS_MERGED_USR=y),
'find lib' does not return the correct result. So, until now,
libpthread*.so* and ld-*.so* were not stripped when 'lib' was a symlink.

We fix this by using 'find lib/' instead of 'find lib'. For consistency
reason, we also do the same change for the 'find' that removes .a and
.la files.

Signed-off-by: Jérôme Pouiller <jezz@sysmic.org>
[Thomas: slightly improved the commit log.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>