Makefile: respect strip exclusions for special libraries

Makefile: respect strip exclusions for special libraries

ld-*.so and libpthread*.so* are not stripped in the same way as other
binaries because some applications need symbols in these libraries in
order to operate correctly.

However, the special handling for these binaries ignores the usual
BR2_STRIP_EXCLUDE_* rules so it is not possible to build an image which
has debugging symbols in these binaries.

Pull out the common find functionality so that we can build two find
commands that re-use the common exclusion rules.

Fix-suggested-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: John Keeping <john@metanate.com>
Tested-by: Matt Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>