objtool wrapper: use absolute path to call the wrapper.

[AttilaFueloep]

Motivation and Context

Older kernel versions run make outside of the build directory. This works since all paths are absolute. Relative paths will fail in such a scenario. See #17526. Follow-up for #17456.

Description

Use an absolute path to the objtool wrapper as well, since the relative path breaks the build on older kernels.

How Has This Been Tested?

CI run on local repo.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Quality assurance (non-breaking change which makes the code more robust against bugs)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.