llext: add build commands and tests for relocatable ELFs

[pillo79]

This PR introduces build support in the llext subsystem for relocatable objects, and a Twister test for validating this PR and #70793, which added llext support for them. It is intended to replace #70991 by providing the same result on the new CMake infrastructure merged in #67997.

It does so by:

• introducing the LLEXT_BINARY_TYPE Kconfig, to select the build type among object files, relocatable files, or shared libraries, effectively making CMake code for llext almost arch-independent. The automatic defaults are for the tested combinations, but this gives developers more freedom to experiment.
• relaxing add_llext_target rules to support multiple source files for the binary types that include a link step;
• adding build support for the relocatable case;
• incorporating the last 2 commits of [DNM] LLEXT: Relocatable test #70991 from @lyakh adding support for zero-based addresses and a relocatable test case.

[lyakh]

Thanks! Tested locally too.

[marc-hb]

You're removing message(FATAL_ERROR "add_llext_target: unsupported architecture"). So isn't that enough to remove the LOADER_BUILD_ONLY test limitation?

[pillo79]

You're removing message(FATAL_ERROR "add_llext_target: unsupported architecture"). So isn't that enough to remove the LOADER_BUILD_ONLY test limitation?

Good idea! I have tried removing the barriers but I get a weird error on x86, native and a53 arches:

In file included from zephyr/include/zephyr/toolchain.h:50,
                 from zephyr/include/zephyr/sys/__assert.h:11,
                 from zephyr/include/zephyr/sys/iterable_sections.h:10,
                 from zephyr/include/zephyr/llext/symbol.h:10,
                 from zephyr/tests/subsys/llext/simple/src/relative_jump_ext.c:15:
zephyr/include/zephyr/toolchain/gcc.h:573:2: error: #error processor architecture not supported
  573 | #error processor architecture not supported
      |  ^~~~~

(and all other llext tests). The compiler is missing autoconf.h; this is due to the fact that on these arches, the build.ninja rules for llext code have no FLAGS = line at all, even though the CMake code is obviously the same as the arches we are testing. This is next level for me as well... 🤷‍♂️

[pillo79]

on these arches, the build.ninja rules for llext code have no FLAGS = line at all, even though the CMake code is obviously the same as the arches we are testing. This is next level for me as well... 🤷‍♂️

blah blah daemons blah blah

Turns out the reason was way clearer - arches where LLEXT is not tested yet do not define an LLEXT_REMOVE_FLAGS variable, so the code turns it into an empty regexp, which promptly results in $<FILTER:...> erasing the whole list of Zephyr flags. 🤦‍♂️

I have this fixed locally, however since the PR is already approved I will open a separate one with this cleanup/refactor as soon as this lands.

[marc-hb]

Turns out the reason was way clearer

Nice! Well done.

since the PR is already approved...

Agreed.

Note dropping LOADER_BUILD_ONLY is the ultimate goal BUT if you have it to keep it for some more time and gradually increase the list of architectures that actually run tests instead then it would be great progress too. A first, 64bits arch passing the tests would be an especially big milestone!