perf tools: Create source symlink in perf object dir

Create a source symlink to the original source in the objdir.

This is similar to what the main kernel build script does.

Committer testing:

  ⬢[acme@toolbox perf-tools-next]$ make O=/tmp/build/$(basename $PWD)/ -C tools/perf install-bin
  <SNIP>
  ⬢[acme@toolbox perf-tools-next]$ ls -la /tmp/build/perf-tools-next/source
  lrwxrwxrwx. 1 acme acme 41 Aug  9 16:26 /tmp/build/perf-tools-next/source -> /home/acme/git/perf-tools-next/tools/perf
  ⬢[acme@toolbox perf-tools-next]$
Signed-off-by: Andi Kleen <ak@linux.intel.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Acked-by: Ian Rogers <irogers@google.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20240807231823.898979-1-ak@linux.intel.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Makefile.perf

@@ -163,6 +163,8 @@ ifneq ($(OUTPUT),)
 # for flex/bison parsers.
 VPATH += $(OUTPUT)
 export VPATH
+# create symlink to the original source
+SOURCE := $(shell ln -sf $(srctree)/tools/perf $(OUTPUT)/source)
 endif
 
 ifeq ($(V),1)