perf: Correct the substitution of python shebangs

perf: Correct the substitution of python shebangs

To make the native python3 always used,

- Use sed one-liner instead
- Add substitution for ${S}/scripts/bpf_helpers_doc.py to fix the
  following warning.

File "/usr/lib/python3.6/sysconfig.py", line 421, in _init_posix
_temp = __import__(name, globals(), locals(), ['build_time_vars'], 0)
ModuleNotFoundError: No module named '_sysconfigdata'

This issue is first reported by Joel Stanley <joel@jms.id.au>
The sed one-liner is credited to Anuj Mittal <anuj.mittal@intel.com>

(From OE-Core rev: 3f93173130a94310255389cfc62c67102a4fb21b)

Signed-off-by: He Zhe <zhe.he@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 3a1a18ba9d28adb5562eabe9ec354f6d93154f5c)
Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>