x86-64/ELF: clear src_mask for all reloc types

x86-64 uses rela relocations. The comment next to the field's declaration
says "Non-zero values for ELF USE_RELA targets should be viewed with
suspicion ..." And indeed the fields being non-zero causes section
contents to be accumulated into the final relocated values in addition to
the relocations' addends, which is contrary to the ELF spec.