xen/build: Add cppcheck and cppcheck-html make rules

xen/build: Add cppcheck and cppcheck-html make rules

cppcheck can be used to check Xen code quality.

To create a report do "make cppcheck" on a built tree adding any options
you added during the process you used to build xen (like CROSS_COMPILE
or XEN_TARGET_ARCH). This will generate an xml report xen-cppcheck.xml.

To create a html report do "make cppcheck-html" in the same way and a
full report to be seen in a browser will be generated in
cppcheck-htmlreport/index.html.

For better results it is recommended to build your own cppcheck from the
latest sources that you can find at [1].
Development and result analysis has been done with cppcheck 2.7.

The Makefile rule is searching for all C files which have been compiled
(ie which have a generated .o file) and is running cppcheck on all of
them using the current configuration of xen so only the code actually
compiled is checked.

A new tool is introduced to merge all cppcheck reports into one global
report including all findings and removing duplicates.

Some extra variables can be used to customize the report:
- CPPCHECK can be used to give the full path to the cppcheck binary to
use (default is to use the one from the standard path).
- CPPCHECK_HTMLREPORT can be used to give the full path to
cppcheck-htmlreport (default is to use the one from the standard path).

This has been tested on several arm configurations (x86 should work but
has not been tested).

[1] https://cppcheck.sourceforge.io/

Signed-off-by: Bertrand Marquis <bertrand.marquis@arm.com>
Signed-off-by: Michal Orzel <michal.orzel@arm.com>
[stefano: add in-code comment on commit]
Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
Tested-by: Stefano Stabellini <sstabellini@kernel.org>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>
Acked-by: Julien Grall <jgrall@amazon.com>