PR26979, Visibility of undefined foo@v1 should constrain foo@@v1

PR26979, Visibility of undefined foo@v1 should constrain foo@@v1

Also, undefined foo should constrain the visibility of foo@@v1 just as
it does for a later plain foo definition.

bfd/
	PR 26979
	* elf-bfd.h (elf_backend_merge_symbol_attribute): Update prototype.
	* elf32-m68hc1x.h (elf32_m68hc11_merge_symbol_attribute): Likewise.
	* elfxx-mips.h (_bfd_mips_elf_merge_symbol_attribute): Likewise.
	* elfxx-x86.h (_bfd_x86_elf_merge_symbol_attribute): Likewise.
	* elf32-m68hc1x.c (elf32_m68hc11_merge_symbol_attribute): Replace
	isym parameter with st_other.  Adjust code.
	* elf64-alpha.c (elf64_alpha_merge_symbol_attribute): Likewise.
	* elf64-ppc.c (ppc64_elf_merge_symbol_attribute): Likewise.
	* elfnn-aarch64.c (elfNN_aarch64_merge_symbol_attribute): Likewise.
	* elfxx-mips.c (_bfd_mips_elf_merge_symbol_attribute): Likewise.
	* elfxx-x86.c (_bfd_x86_elf_merge_symbol_attribute): Likewise.
	* elflink.c (elf_merge_st_other): Likewise.
	(_bfd_elf_merge_symbol, elf_link_add_object_symbols): Adjust to suit.
	(_bfd_elf_copy_link_hash_symbol_type): Likewise.
	(_bfd_elf_add_default_symbol): Merge st_other from undecorated
	symbol and @VER symbol to @@VER symbol.
ld/
	* testsuite/ld-elf/pr26979a.s,
	* testsuite/ld-elf/pr26979b.s,
	* testsuite/ld-elf/pr26979c.s,
	* testsuite/ld-elf/pr26979.ver,
	* testsuite/ld-elf/pr26979a.d,
	* testsuite/ld-elf/pr26979b.d: New tests.