tools build: Fix feature detect clean for out of source builds

Currently the clean target when using O= isn't cleaning the feature
detect output. This is because O= and OUTPUT= are set to canonical
paths. For example in tools/perf/Makefile:

  FULL_O := $(shell cd $(PWD); readlink -f $(O) || echo $(O))

This means that OUTPUT ends in a / and most usages prepend it to a file
without adding an extra /. This line that was changed adds an extra /
before the 'feature' folder but not to the end, resulting in a clean
command like this:

  rm -f /tmp/build//featuretest-all.bin ...

After the change the clean command looks like this:

  rm -f /tmp/build/feature/test-all.bin ...

Fixes: 762323eb

 ("perf build: Move feature cleanup under tools/build")
Signed-off-by: James Clark <james.clark@arm.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: http://lore.kernel.org/lkml/20210816130705.1331868-1-james.clark@arm.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>