csky: Support PC relative diff relocation

csky: Support PC relative diff relocation

Define DIFF_EXPR_OK to Support PC relative diff relocation,
and add CKCORE_PCREL32 relocation process

bfd/
        * elf32-csky.c (csky_elf_howto_table): Fill special_function of
        R_CKCORE_PCREL32.
        (csky_elf_relocate_section): Add R_CKCORE_PCREL32 process.
gas/
        * config/tc-csky.c (md_apply_fix): Transmit
        BFD_RELOC_32_PCREL to BFD_RELOC_CKCORE_PCREL32.
        (tc_gen_reloc): Trasmit BFD_RELOC_CKCORE_ADDR32 to
        BFD_RELOC_CKCORE_PCREL32 while pc-relative.
        * config/tc-csky.h (DIFF_EXPR_OK): Define to enable PC relative
        diff relocs.