LoongArch: bfd: Add support for linker relaxation.

Add relax support and related relocs in bfd.

bfd/ChangeLog:

	* bfd-in2.h: Add relocs related to relax.
	* elfnn-loongarch.c (struct loongarch_elf_link_hash_table): New integer
	pointer (data_segment_phase) to monitor the data segment phase.
	(loongarch_elf_check_relocs): Swap B21/B26 reloc sequence.
	(loongarch_elf_adjust_dynamic_symbol): Fix code format.
	(loongarch_reloc_rewrite_imm_insn): Fix function call.
	(perform_relocation): Handle new relocs related to relax.
	(RELOCATE_CALC_PC32_HI20): Fix code format.
	(RELOCATE_CALC_PC64_HI32): Likewise.
	(loongarch_elf_relocate_section): Handle new relocs related to relax.
	(loongarch_relax_delete_bytes): New function.
	(loongarch_relax_pcala_addi): Likewise.
	(loongarch_relax_pcala_ld): Likewise.
	(bfd_elfNN_loongarch_set_data_segment_info): Likewise.
	(loongarch_relax_align): Likewise.
	(loongarch_elf_relax_section): Likewise.
	(bfd_elfNN_bfd_relax_section): New macro define.
	* elfxx-loongarch.c (reloc_bits): New bfd point parameter.
	(reloc_bits_b16): Likewise.
	(reloc_bits_b21): Likewise.
	(reloc_bits_b26): Likewise.
	(loongarch_adjust_reloc_bitsfield): Likewise.
	(reloc_bits_pcrel20_s2): New function.
	(loongarch_elf_add_sub_reloc): Likewise.
	(loongarch_elf_add_sub_reloc_uleb128): Likewise.
	(loongarch_write_unsigned_leb128): New function.
	* elfxx-loongarch.h (loongarch_adjust_reloc_bitsfield): New bfd point
	parameter.
	(bfd_elf32_loongarch_set_data_segment_info): New declare.
	(bfd_elf64_loongarch_set_data_segment_info): Likewise.
	(loongarch_write_unsigned_leb128): Likewise.
	* libbfd.h: Add relocs related to relax.
	* reloc.c: Add relocs related to relax.