[PATCH 2/2] RISC-V: Merge ELF attribute for ld

[Kito Cheng]

This patch implement the RISC-V ELF attribute merge, only implement
exact arch version match now, we'll implement different merge policy
later.

ChangeLog:
bfd/
* elfnn-riscv.c (in_subsets): New variable.
(out_subsets): Likewise.
(merged_subsets): Likewise.
(riscv_std_ext_p): Likewise.
(riscv_non_std_ext_p): Likewise.
(riscv_std_sv_ext_p): Likewise.
(riscv_non_std_sv_ext_p): Likewise.
(riscv_version_mismatch): Likewise.
(riscv_i_or_e_p): Likewise.
(riscv_merge_std_ext): Likewise.
(riscv_merge_non_std_and_sv_ext): Likewise.
(riscv_merge_arch_attr_info): Likewise.
(riscv_merge_attributes): Likewise.
(riscv_elf_modify_segment_map): Likewise.
(riscv_elf_additional_program_headers): Likewise.
(_bfd_riscv_elf_merge_private_bfd_data): Merge attribute.
(elf_backend_additional_program_headers): Define as
riscv_elf_additional_program_headers.
(elf_backend_modify_segment_map): Define as
riscv_elf_modify_segment_map.

binutils/
* readelf.c (get_riscv_segment_type) New function.
(get_segment_type): Add handler for RISC-V.

include/
* elf/riscv.h (PT_RISCV_ATTRIBUTES): Define.

ld/
* testsuite/ld-elf/orphan-region.d: XFAIL for RISC-V, because add new
section.
* testsuite/ld-scripts/size.exp: Unsupported size-2 for RISC-V,
because RISC-V has new segment in PHDR.
* testsuite/ld-riscv-elf/ld-riscv-elf.exp: Add new tests.
* testsuite/ld-riscv-elf/attr-merge-arch-01.d: New test.
* testsuite/ld-riscv-elf/attr-merge-arch-01a.s: Likewise.
* testsuite/ld-riscv-elf/attr-merge-arch-01b.s: Likewise.
* testsuite/ld-riscv-elf/attr-merge-arch-02.d: Likewise.
* testsuite/ld-riscv-elf/attr-merge-arch-02a.s: Likewise.
* testsuite/ld-riscv-elf/attr-merge-arch-02b.s: Likewise.
* testsuite/ld-riscv-elf/attr-merge-arch-03.d: Likewise.
* testsuite/ld-riscv-elf/attr-merge-arch-03a.s: Likewise.
* testsuite/ld-riscv-elf/attr-merge-arch-03b.s: Likewise.
* testsuite/ld-riscv-elf/attr-merge-arch-failed-01.d: Likewise.
* testsuite/ld-riscv-elf/attr-merge-arch-failed-01a.s: Likewise.
* testsuite/ld-riscv-elf/attr-merge-arch-failed-01b.s: Likewise.
* testsuite/ld-riscv-elf/attr-merge-priv-spec-a.s: Likewise.
* testsuite/ld-riscv-elf/attr-merge-priv-spec-b.s: Likewise.
* testsuite/ld-riscv-elf/attr-merge-priv-spec.d: Likewise.
* testsuite/ld-riscv-elf/attr-merge-stack-align-a.s: Likewise.
* testsuite/ld-riscv-elf/attr-merge-stack-align-b.s: Likewise.
* testsuite/ld-riscv-elf/attr-merge-stack-align-failed-a.s: Likewise.
* testsuite/ld-riscv-elf/attr-merge-stack-align-failed-b.s: Likewise.
* testsuite/ld-riscv-elf/attr-merge-stack-align-failed.d: Likewise.
* testsuite/ld-riscv-elf/attr-merge-stack-align.d: Likewise.
* testsuite/ld-riscv-elf/attr-merge-strict-align-01.d: Likewise.
* testsuite/ld-riscv-elf/attr-merge-strict-align-01a.s: Likewise.
* testsuite/ld-riscv-elf/attr-merge-strict-align-01b.s: Likewise.
* testsuite/ld-riscv-elf/attr-merge-strict-align-02.d: Likewise.
* testsuite/ld-riscv-elf/attr-merge-strict-align-02a.s: Likewise.
* testsuite/ld-riscv-elf/attr-merge-strict-align-02b.s: Likewise.
* testsuite/ld-riscv-elf/attr-merge-strict-align-03.d: Likewise.
* testsuite/ld-riscv-elf/attr-merge-strict-align-03a.s: Likewise.
* testsuite/ld-riscv-elf/attr-merge-strict-align-03b.s: Likewise.
* testsuite/ld-riscv-elf/attr-merge-strict-align-04.d: Likewise.
* testsuite/ld-riscv-elf/attr-merge-strict-align-04a.s: Likewise.
* testsuite/ld-riscv-elf/attr-merge-strict-align-04b.s: Likewise.
* testsuite/ld-riscv-elf/attr-merge-strict-align-05.d: Likewise.
* testsuite/ld-riscv-elf/attr-merge-strict-align-05a.s: Likewise.
* testsuite/ld-riscv-elf/attr-merge-strict-align-05b.s: Likewise.