NDS32: Code refactoring of relaxation.

NDS32: Code refactoring of relaxation.

Refactor each relaxation pattern to raise the maintainability.
In origin, all patterns is analysed in nds32_elf_relax_section,
so it is hard to debug and maintain.  Therefore, we classify all
patterns into different functions in this patch.
Moreover, we adjust all optimizations into nds32_elf_relax_section
to take these optimizations in turn.  This can promise all relaxation
being done after calling gld${EMULATION_NAME}_after_allocation.