scripts/pahole-flags.sh: Parse DWARF and generate BTF with multithreading.

Pass a "-j" argument to pahole if possible to reduce the time of
generating BTF info.

Since v1.22, pahole can parse DWARF and generate BTF with
multithreading to speed up the conversion.  It will reduce the overall
build time of the kernel for seconds.

v3 fixes whitespaces and improves the commit description.
v2 checks the version of pahole to enable multithreading only if possible.

[v2] https://lore.kernel.org/bpf/20220216193431.2691015-1-kuifeng@fb.com/
[v1] https://lore.kernel.org/bpf/20220216004616.2079689-1-kuifeng@fb.com/



Signed-off-by: Kui-Feng Lee <kuifeng@fb.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20220217175427.649713-1-kuifeng@fb.com