perf: fix python scripts QA errors

perf: fix python scripts QA errors

Two QA warnings were being generated from perf, due to the location of
python scripts changing in the kernel build:

WARNING: perf-1.0-r9 do_package_qa: QA Issue:
  /usr/libexec/perf-core/scripts/python/call-graph-from-postgresql.py contained
  in package perf-python requires /usr/bin/python2, but no providers found in RDEPENDS_perf-python? [file-rdeps]

WARNING: QA Issue: /usr/libexec/perf-core/tests/attr.py_perf contained in package
  perf requires /usr/bin/python, but no providers found in its RDEPENDS [file-rdeps]

By adding libexec dir to both the tests and scripts packaging, we get the
appropriate RDEPENDS on the packages that actually contain the python scripts
(without making perf-core depend on python).

We also tweak any python scripts to use '/usr/bin/env python', rather than a
version specific python, since it won't be provided and generate a QA error.

[YOCTO #8991]

(From OE-Core rev: d7888b6d7fd1a1930013f3d4e31a2522038cdc62)

Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>