RISC-V: Implement support for big endian targets.

RISC-V: Implement support for big endian targets.

RISC-V instruction/code is always little endian, but data might be
big-endian.  Therefore, we can not use the original bfd_get/bfd_put
to get/put the code for big endian targets.  Add new riscv_get_insn
and riscv_put_insn to always get/put code as little endian can resolve
the problem.  Just remember to update them once we have supported
the 48-bit/128-bit instructions in the future patches.

bfd/
    * config.bfd: Added targets riscv64be*-*-*, riscv32be*-*-* and
    riscvbe*-*-*.  Also added riscv_elf[32|64]_be_vec.
    * configure.ac: Handle riscv_elf[32|64]_be_vec.
    * configure: Regenerate.
    * elfnn-riscv.c: Include <limits.h> and define CHAR_BIT for
    riscv_is_insn_reloc.
    (riscv_get_insn): RISC-V instructions are always little endian, but
    bfd_get may be used for big-endian, so add new riscv_get_insn to handle
    the insturctions.
    (riscv_put_insn): Likewsie.
    (riscv_is_insn_reloc): Check if we are relocaing an instruction.
    (perform_relocation): Call riscv_is_insn_reloc to decide if we should
    use riscv_[get|put]_insn or bfd_[get|put].
    (riscv_zero_pcrel_hi_reloc): Use riscv_[get|put]_insn, bfd_[get|put]l32
    or bfd_[get|put]l16 for code.
    (riscv_elf_relocate_section): Likewise.
    (riscv_elf_finish_dynamic_symbol): Likewise.
    (riscv_elf_finish_dynamic_sections): Likewise.
    (_bfd_riscv_relax_call): Likewise.
    (_bfd_riscv_relax_lui): Likewise.
    (_bfd_riscv_relax_align): Likewise.
    (_bfd_riscv_relax_pc): Likewise.
    (riscv_elf_object_p): Handled for big endian.
    (TARGET_BIG_SYM, TARGET_BIG_NAME): Defined.
    * targets.c: Add riscv_elf[32|64]_be_vec.
    (_bfd_target_vector): Likewise.

gas/
    * config/tc-riscv.c (riscv_target_format): Add elf64-bigriscv and
    elf32-bigriscv.
    (install_insn): Always write instructions as little endian.
    (riscv_make_nops): Likewise.
    (md_convert_frag_branch): Likewise.
    (md_number_to_chars): Write data in target endianness.
    (options, md_longopts): Add -mbig-endian and -mlittle-endian options.
    (md_parse_option): Handle the endian options.
    * config/tc-riscv.h: Only define TARGET_BYTES_BIG_ENDIAN if not
    already defined.
    * configure.tgt: Added riscv64be*, riscv32be*, riscvbe*.

ld/
    * configure.tgt: Added riscvbe-*-*, riscv32be*-*-*, riscv64be*-*-*,
    riscv32be*-*-linux*, and riscv64be*-*-linux*.
    * Makefile.am: Added eelf32briscv.c, eelf32briscv_ilp32f.c and
    eelf32briscv_ilp32.c.
    * Makefile.in: Regenerate.
    * emulparams/elf32briscv.sh: Added.
    * emulparams/elf32briscv_ilp32.sh: Likewise.
    * emulparams/elf32briscv_ilp32f.sh: Likewise.
    * emulparams/elf64briscv.sh: Likewise.
    * emulparams/elf64briscv_lp64.sh: Likewise.
    * emulparams/elf64briscv_lp64f.sh: Likewise.