Makefile: do not try to strip inexisting file when stripping libpthread

libpthread.so has a special stripping condition to preserve parts of
it that are needed for debugging. However, due to the usage of 'xargs'
instead of 'xargs -r', the strip command is executed regardless of
whether a libpthread.so file is found or not. This leads to a big
error message being displayed in static-only builds, because strip is
executed without a file argument. Thanks to the '|| true', the build
continues, but still shows a big error message in the middle, which is
not nice.

By using 'xargs -r', we avoid the strip command from being executed
when 'find' doesn't find any match. We also remove the '|| true' to
catch other real errors.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>