For now llext supports a very limited number of architectures. This
restriction is enforced by add_llext_target() in CMake at configuration
time.
Add a new `LOADER_BUILD_ONLY` conditional in tests/subsys/llext/simple/
and a new `llext.simple.loader_build`, `build_only` test that does not
invoke `add_llext_target()` and only compiles the llext framework code.
This helps find and fix bugs in `subsys/llext/*.c` and make it ready to
be used when add_llext_target() limitations are lifted.
Note this is pure `tests/` change without any change in the actual llext
framework code. The existing test is only modified to conditionally
invoke add_llext_target().
Signed-off-by: Marc Herbert <marc.herbert@intel.com>
1408d1e5b8