tests/subsys/llext/simple/llext.simple.loader_build fails to build for native_posix in main

[aescolar]

Describe the bug
The test tests/subsys/llext/simple/llext.simple.loader_build fails to build for native_posix in main

11c350e changed a linker script snippet to set no_syscall_impl (in section llext_no_syscall_impl) hard in address 0, but in a way that sets the address of any further section/symbols after that until the current address is set to something else.

Side notes

• Is this llext_no_syscall_impl needed even if USERSPACE is not enabled?
• Note: native_posix is deprecated, so maybe just filtering it is an option. But still, it does not feel too nice to have sideffects in snippets.
• Just doing this (assuming the current way of doing things is necessary) would avoid the issue:

+       silly = .;
        SECTION_PROLOGUE(llext_no_syscall_impl, 0 (COPY), )
        {
          *(llext_no_syscall_impl)
        }
+       . = silly;

To Reproduce
Steps to reproduce the behavior:

1. mkdir build; cd build
2. cmake -GNinja -DBOARD=native_posix ../tests/subsys/llext/simple/ && ninja
3. ninja
4. See error

Expected behavior
No build failures in enabled platforms.

Impact
2 failures in weekly CI (for native_posix and native_posix//64)
https://github.com/zephyrproject-rtos/zephyr/runs/32564635154
(note there is some other platforms failing this test)

Logs and console output

/usr/bin/ld.bfd: zephyr/zephyr.elf: error: PHDR segment not covered by LOAD segment
collect2: error: ld returned 1 exit status

Environment (please complete the following information):

• OS: Ubuntu 24.04
• Toolchain (e.g Zephyr SDK, ...): gcc 14.2
• Commit SHA or Version used: Main since 11c350e

Additional context
It fails to build since 11c350e

[aescolar]

CC @ edersondisouza @pillo79

[pillo79]

@aescolar definitely an unwanted side effect. Thanks for pinging and providing a workaround, will prepare a PR right away.

EDIT: The symbol is useful even without USERSPACE, since it is assigned to the _impl part of all defined-but-unimplemented syscalls, and finding a needed symbol set to NULL stops the llext loading process.