kbuild: compress the kernel with pigz if available

The gzip step of a kernel build is very lengthy, especially for larger
builds such as allmodconfig.

gzip itself cannot be run in parallel, however an alternative tool exists
that can, providing the same feature set as gzip itself - pigz - which
works as a drop-in replacement.

On a 128-core Threadripper, gzip -9 of a 36 MiB x86-64 vmlinux.bin takes
1.6s, and with pigz it takes 0.09s, so the performance increase is
significant.

Therefore introduce the KPGZIP build variable which specifies the
compressor to use for the kernel image and default it to pigz if available,
or KGZIP otherwise (which itself defaults to gzip).

Modules, packaging and the embedded kernel configuration (CONFIG_IKCONFIG)
continue to use KGZIP which defaults to a single-threaded gzip instance.

Module compression is run in parallel by make, so pigz offers no benefit
there, packaging sits outside the build and the embedded configuration is
too small to matter.

pigz is executed using scripts/jobserver-exec, which reserves the job slots
make has free - necessary, as otherwise pigz would just grab unbounded
CPUs.

The kernel image compression step is always performed at the end of the
build so should typically have all job slots available.

pigz always provides byte-for-byte identical output across runs regardless
of thread count, but pigz and gzip are not guaranteed to generate the same
binary output, so reproducible builds must use the same tools consistently.

Also update Documentation/kbuild/kbuild.rst to describe KPGZIP and
Documentation/process/changes.rst to list the optional pigz dependency.

While there, add entries for the other compressor programs.

Finally, update scripts/ver_linux to include the pigz version there.

Every x86 build ends with the compression of vmlinux.bin, 36MB for
defconfig and over 200MB for allmodconfig, no-op builds are unchanged.

Whole build, 128-thread Threadripper 9980X, best of N runs:

                                         before   after     delta
                                         -------------------------------
  x86 defconfig, touch mm/vma.c, gcc        7.7s     6.0s     -1.7s (-22%)
  x86 defconfig, touch mm/vma.c, clang      6.9s     5.4s     -1.5s (-21%)
  x86 defconfig, clean, gcc                28.2s    26.3s     -1.8s (-6%)
  x86 defconfig, clean, clang              28.7s    27.2s     -1.5s (-5%)
  x86 allmodconfig, touch mm/vma.c, gcc    23.7s    15.4s     -8.3s (-35%)
  x86 allmodconfig, touch mm/vma.c, clang  22.5s    15.6s     -7.0s (-31%)
  x86 allmodconfig, clean, gcc            294.1s   278.6s     -15.5s (-5%)
  x86 allmodconfig, clean, clang          283.9s   266.8s     -17.1s (-6%)

Suggested-by: Kees Cook <kees@kernel.org>
Link: https://zlib.net/pigz/
Assisted-by: LLM
Reviewed-by: Manuel Ebner <manuelebnerli@mailbox.org>
Reviewed-by: Kees Cook <kees@kernel.org>
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Link: https://patch.msgid.link/20260923-build-speedup-v4-22-73128809a4a4@kernel.org
Signed-off-by: Nathan Chancellor <nathan@kernel.org>

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