asan: heap buffer overflow elf_link_add_to_first_hash

Seen on mmix.
mmix  +FAIL: ld-misc/defsym1
mmix  +FAIL: sysroot-prefix common plain -Lpath, quoted
mmix  +FAIL: sysroot-prefix common plain -Lpath, unquoted
mmix  +FAIL: sysroot-prefix common full-path, quoted
mmix  +FAIL: sysroot-prefix common full-path, unquoted
mmix  +FAIL: sysroot-prefix common plain =-prefixed with empty, quoted
mmix  +FAIL: sysroot-prefix common plain =-prefixed with empty, unquoted
mmix  +FAIL: sysroot-prefix common plain $SYSROOT-prefixed with empty, quoted
mmix  +FAIL: sysroot-prefix common plain $SYSROOT-prefixed with empty, unquoted
mmix  +FAIL: sysroot-prefix common plain =-prefixed -Lpath, quoted
mmix  +FAIL: sysroot-prefix common plain =-prefixed -Lpath, unquoted
mmix  +FAIL: sysroot-prefix common plain $SYSROOT-prefixed -Lpath, quoted
mmix  +FAIL: sysroot-prefix common plain $SYSROOT-prefixed -Lpath, unquoted
mmix  +FAIL: sysroot-prefix common full-path =-prefixed without, quoted
mmix  +FAIL: sysroot-prefix common full-path =-prefixed without, unquoted
mmix  +FAIL: sysroot-prefix common full-path $SYSROOT-prefixed without, quoted
mmix  +FAIL: sysroot-prefix common full-path $SYSROOT-prefixed without, unquoted

==3746597==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x6070000007a0 at pc 0x56d87b0d1a40 bp 0x7fffb1629bf0 sp 0x7fffb1629be0
READ of size 8 at 0x6070000007a0 thread T0
    #0 0x56d87b0d1a3f in elf_link_add_to_first_hash /home/alan/src/binutils-gdb/bfd/elflink.c:4312

mmix uses bfd_link_generic_hash_table.

	* elflink.c (_bfd_elf_archive_symbol_lookup): Dont use first_hash
	unless the hash table is bfd_link_elf_hash_table.
	(elf_link_add_archive_symbols): Likewise.

bfd/elflink.c

@@ -6116,7 +6116,8 @@ _bfd_elf_archive_symbol_lookup (bfd *abfd,
     {
       /* Add this symbol to first hash if this archive has the first
 	 definition.  */
-      elf_link_add_to_first_hash (abfd, info, name, false);
+      if (is_elf_hash_table (info->hash))
+	elf_link_add_to_first_hash (abfd, info, name, false);
       return h;
     }
 
@@ -6258,6 +6259,8 @@ elf_link_add_archive_symbols (bfd *abfd, struct bfd_link_info *info)
 		/* Symbol must be defined.  Don't check it again.  */
 		included[i] = true;
 
+	      if (!is_elf_hash_table (info->hash))
+		continue;
 	      /* Ignore the archive if the symbol isn't defined in a
 		 shared object.  */
 	      if (!((struct elf_link_hash_entry *) h)->def_dynamic)