tests: llext: compile architectures not supported yet

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>