bfd/ELF: mark functions exposed to ld as non-private

As non-private functions, these shouldn't have "_bfd_" prefixes,
but merely "bfd_" ones:
- _bfd_elf_size_group_sections(),
- _bfd_elf_match_sections_by_type(),
- _bfd_elf_strtab_{str,len}(),
- _bfd_elf_map_sections_to_segments(),
- _bfd_elf_tls_setup().

ld/ldlang.c

@@ -6536,7 +6536,7 @@ lang_size_sections_1
   return dot;
 }
 
-/* Callback routine that is used in _bfd_elf_map_sections_to_segments.
+/* Callback routine that is used in bfd_elf_map_sections_to_segments.
    The BFD library has set NEW_SEGMENT to TRUE iff it thinks that
    CURRENT_SECTION and PREVIOUS_SECTION ought to be placed into different
    segments.  We are allowed an opportunity to override this decision.  */