[AArch64] Allow COPY relocation elimination

  As discussed at the PR, this patch tries to avoid COPY relocation generation
and propagate the original relocation into runtime if it was relocating on
writable section.  The ELIMINATE_COPY_RELOCS has been set to true and it's
underlying infrastructure has been improved so that the COPY reloc elimination
at least working on absoluate relocations (ABS64) after this patch.

bfd/
        PR ld/21532
        * elfnn-aarch64.c (ELIMINATE_COPY_RELOCS): Set to 1.
        (elfNN_aarch64_final_link_relocate): Also propagate relocations to
        runtime for copy relocation elimination cases.
        (alias_readonly_dynrelocs): New function.
        (elfNN_aarch64_adjust_dynamic_symbol): Keep the dynamic relocs instead
        of generating copy relocation if it is not against read-only sections.
        (elfNN_aarch64_check_relocs): Likewise.

ld/
        * testsuite/ld-aarch64/copy-reloc-eliminate.d: New test.
        * testsuite/ld-aarch64/copy-reloc-exe-eliminate.s: New test source file.
        * testsuite/ld-aarch64/aarch64-elf.exp: Run new testcase.