linux(4): Fix gcc buld.

gcc failed as it didn't inlined the builtins and generates calls to
the libgcc, ld can't find libgcc as cross-toolchain libgcc is not installed.
To avoid this add internal vDSO ffs functions without optimized builtins.

Reported by: jhb
MFC after: 2 weeks

(cherry picked from commit f33794014468562100f6f02ab8147b58bd9f82c0)