As discussed at https://forums.gentoo.org/viewtopic-p-7745434.html and https://forums.gentoo.org/viewtopic-p-7685464.html, compilation of hardened sources will fail at 'Documentation/vDSO/vdso_standalone_test_x86' if CONFIG_BUILD_DOCSRC=y is set.
*** This bug has been marked as a duplicate of bug 567732 ***