Instead of setting XCC_USE_CLANG=1, this patch adds xcc-clang toolchain
that is basically xcc using the clang compiler.
Initially, the new toolchain simply includes files from current xcc
toolchain and (re)sets some variables. This should be a more scalable
approach to diverge the toolchains in the future than placing
"if($ENV{XCC_USE_CLANG})" at several places.
It should also help to filter tests that run (or not) exclusively with
the clang variant of XCC on twister.
The XCC_USE_CLANG flag is documented as deprecated, and a message is
emitted during build if still in use. Its new behaviour is to instruct
Zephyr to use `xcc-clang` toolchain.
Signed-off-by: Ederson de Souza <ederson.desouza@intel.com>
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