bfd/ELF: fix BFD library build --enable-shared

The patch series that added support for Object Attributes v2 introduced
regressions when building the BFD library as a shared object.

Incorrect usages of ATTRIBUTE_HIDDEN caused the following link-time errors:

/usr/bin/ld: config/obj-elf-attr.o: in function `obj_attr_v2_record':
obj-elf-attr.c: undefined reference to `bfd_elf_obj_attr_v2_init'
obj-elf-attr.c: undefined reference to `_bfd_obj_attr_v2_find_by_tag'
obj-elf-attr.c: undefined reference to `obj_attr_v2_t_append'
/usr/bin/ld: config/obj-elf-attr.o: in function `obj_attr_v2_subsection_record':
obj-elf-attr.c: undefined reference to `obj_attr_subsection_v2_t_append'
obj-elf-attr.c: undefined reference to `obj_attr_subsection_v2_t_remove'
obj-elf-attr.c: undefined reference to `obj_attr_subsection_v2_t_append'

This patch fixes the symbols visibility so that the BFD library links
correctly when built with --enable-shared.
The ATTRIBUTE_HIDDEN annotations were removed from bfd_elf_obj_attr_v2_init
and _bfd_obj_attr_v2_find_by_tag, and _bfd_obj_attr_v2_find_by_tag was renamed
to reflect the belonging to the public BFD API using the 'bfd_' prefix. The
doubly linked list helpers remain hidden and are instead exposed through wrapper
functions.