RISC-V: Fix that IRELATIVE relocs may be inserted to the wrong place.

For the ifunc symbol, which is referenced by GOT rather than PLT relocs,
we should add the dynamic reloc (usually IRELATIVE) into the .rel.iplt
when generating the static executable.  But if we use riscv_elf_append_rela
to add the dynamic relocs into .rela.iplt, this may cause the overwrite
problem.

The reason is that we don't handle the `reloc_index` of .rela.iplt, but
the riscv_elf_append_rela adds the relocs to the place that are calculated
from the reloc_index (in seqential).  Therefore, we may overwrite the
dynamic relocs when the `reloc_index` of .rela.iplt isn't handled correctly.

One solution is that we can add these dynamic relocs (GOT ifunc) from
the last of .rela.iplt section.  But I'm not sure if it is the best way.

	bfd/
	* elfnn-riscv.c (riscv_elf_link_hash_table): Add last_iplt_index.
	(riscv_elf_size_dynamic_sections): Initialize the last_iplt_index.
	(riscv_elf_relocate_section): Use riscv_elf_append_rela.
	(riscv_elf_finish_dynamic_symbol): If the use_elf_append_rela is
	false, then we should add the dynamic relocs from the last of
	the .rela.iplt, and don't use the riscv_elf_append_rela to add.

	ld/
	* testsuite/ld-riscv-elf/ifunc-plt-got-overwrite.s: New testcase.
	* testsuite/ld-riscv-elf/ifunc-plt-got-overwrite.d: Likewise.
	* testsuite/ld-riscv-elf/ifunc-plt-got-overwrite-exe.rd: Likewise.
	* testsuite/ld-riscv-elf/ifunc-plt-got-overwrite-pic.rd: Likewise.
	* testsuite/ld-riscv-elf/ifunc-plt-got-overwrite-pie.rd: Likewise.
	* testsuite/ld-riscv-elf/ld-riscv-elf.exp: Updated.

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