perf.bb: fix multilib build

perf.bb: fix multilib build

Fixed:
$ bitbake perf lib32-perf
ERROR: QA Issue: lib32-perf: Files/directories were installed but not shipped
  /usr/lib64
  /usr/lib64/traceevent
  /usr/lib64/traceevent/plugins
  /usr/lib64/traceevent/plugins/plugin_function.so
  /usr/lib64/traceevent/plugins/plugin_scsi.so
  /usr/lib64/traceevent/plugins/plugin_hrtimer.so
  /usr/lib64/traceevent/plugins/plugin_kmem.so
  /usr/lib64/traceevent/plugins/plugin_jbd2.so
  /usr/lib64/traceevent/plugins/plugin_xen.so
  /usr/lib64/traceevent/plugins/plugin_mac80211.so
  /usr/lib64/traceevent/plugins/plugin_cfg80211.so
  /usr/lib64/traceevent/plugins/plugin_sched_switch.so
  /usr/lib64/traceevent/plugins/plugin_kvm.so [installed-vs-shipped]

The perf.do_configure edits kernel's
${STAGING_KERNEL_DIR}/tools/perf/config/Makefile, there would be
problems since kernel doesn't have multilib, and the build result is
undetermined.

Previously, the sed command changed libdir to /usr/lib64 (or 32) in the
Makefile, so the build result was different if we build perf (64) first
or lib32-perf first.

Use the weak assignment "libdir ?=" to instead of "libdir =" will fix
the problem since the multilib builds are in different processes, and
they won't affect each other any more.

The sed command will match both $(prefix)/$(lib) and $(prefix)/lib since
the Makefile may has been modified before this patch.

(From OE-Core rev: c62d693e1341be4afcaaeb1ee37360ecac71f46a)

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>