llvm: add PACKAGECONFIG[optviewer]

llvm: add PACKAGECONFIG[optviewer]

Currently the CMake configuration for llvm finds the python pygments and
pyyaml if they are installed on the host. This makes builds
non-reproducible across different machines, some having those modules
installed and some others not.

This has been causing a 'reproducible' failure on one of the autobuilders:

  AssertionError: The following deb packages are missing or different and not in exclusion list: /home/pokybuild/yocto-worker/reproducible/build/build-st/reproducibleB/tmp/deploy/deb/./core2-64/llvm-dev_14.0.4-r0_amd64.deb
  The following ipk packages are missing or different and not in exclusion list: /home/pokybuild/yocto-worker/reproducible/build/build-st/reproducibleB/tmp/deploy/ipk/./core2-64/llvm-dev_14.0.4-r0_core2-64.ipk
  The following rpm packages are missing or different and not in exclusion list: /home/pokybuild/yocto-worker/reproducible/build/build-st/reproducibleB/tmp/deploy/rpm/./core2_64/llvm-dev-14.0.4-r0.core2_64.rpm

Fix by adding an explicit PACKAGECONFIG to control the
LLVM_HAVE_OPT_VIEWER_MODULES option in CMake. If unset, it forces CMake to
ignore pygments and pyyaml, no matter it they are available on the host. If
set, it DEPENDS on the pygments and pyyaml modules so that CMake will find
and use them.

Based on suggestions from Stefan and Markus -- thanks.

Proposed-by: "Stefan Herbrechtsmeier" <stefan.herbrechtsmeier-oss@weidmueller.com>
Proposed-by: "Markus Volk" <f_l_k@t-online.de>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>