kldxref: Fix for 32-bit powerpc

R_PPC_RELATIVE lost its 32-bit handling in the libelf conversion.

Reported by: bapt
Fixes: 0299afdff145 ("kldxref: Make use of libelf to be a portable cross tool")

(cherry picked from commit d1ce87ae0ddca413b098b6be8f5d3b4da87422ef)