package_rpm: restrict rpm to 4 threads

TL;DR version:

with this, and the previous compression level changes
I am seeing drastic speedups in package_write_rpm completion times:

webkitgtk goes from 78 seconds to 37 seconds
glibc-locale goes from 399 seconds to 58 seconds (!)

The long version:

rpm uses multithreading for two purposes:

- spawning compressors (which are nowadays themselves
multi-threaded, so the feature is not as useful as it once
was)
- parallel file classification

While the former behaves well on massively parallel CPUs
(it was written and verified here :), the latter was then added
by upstream and only benchmarked on their very old, slow laptop,
apparently:
https://github.com/rpm-software-management/rpm/commit/41f0e214f2266f02d6185ba11f797716de8125d4

On anything more capable it starts showing pathologic behavior,
presumably from spawning massive amount of very short-lived threads,
and then having to synchronize them. For example classifying glibc-locale
takes
5m20s with 256 threads (default on my machine!)
1m49s with 64 threads
59s with 16 threads
48s with 8 threads

Even a more typical recipe like webkitgtk is affected:
47s with 256 threads
32s with 64 threads
27s with 16 or 8 threads

I have found that the optimal amount is actually four: this also
means that only four compressors are running at a time, but
as they're themselves using threads, and typical recipes are dominated
by just two or three large packages, this does not affect overall
completion time.

(From OE-Core rev: 8cbf3a2cddbff1728bd3d1382c3fec00b604501e)

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Steve Sakoman <steve@sakoman.com>

meta/classes-global/package_rpm.bbclass

@@ -705,6 +705,7 @@ python do_package_rpm () {
     cmd = cmd + " --define '_use_internal_dependency_generator 0'"
     cmd = cmd + " --define '_binaries_in_noarch_packages_terminate_build 0'"
     cmd = cmd + " --define '_build_id_links none'"
+    cmd = cmd + " --define '_smp_ncpus_max 4'"
     cmd = cmd + " --define '_source_payload %s'" % rpmbuild_compmode
     cmd = cmd + " --define '_binary_payload %s'" % rpmbuild_compmode
     cmd = cmd + " --define 'clamp_mtime_to_source_date_epoch 1'"