It is needed to generate full Multiboot (GRUB) compiliant OS
kernels with FASM. The only thing I need from FASM is to make
an ELF executeable with specified Image base (add 'at' feature) and relocation info
included, well a simple ELF generated by GNU's
LD linker is an example.

Thanks.

Relocation info is needed for ELF object, not necessarily ELF
executable. If you need an executable containing some object structures, just use
ld with the objects generated by fasm.