PR22626, invalid dynindx used for dynamic relocs against section syms

PR22626, invalid dynindx used for dynamic relocs against section syms

_bfd_elf_link_renumber_dynsyms is called twice by the linker.  The
first call in bfd_elf_size_dynamic_sections is just to answer the
question as to whether there are there any dynamic symbols.  The
second call in bfd_elf_size_dynsym_hash_dynstr sets the st_shndx value
that dynamic symbols will have.  strip_excluded_output_sections is
called between these two calls.  So sections seen on the first
_bfd_elf_link_renumber_dynsyms pass might differ from those seen on
the second pass.  Unfortunately, that can result in a stripped
section's dynamic symbol being assigned a dynindx on the first pass
but not corrected to the final value (of zero, ie. not dynamic) on the
second pass.  PowerPC, x86, mips, and most other targets that emit
dynamic section symbols, just test that section symbol dynindx is
non-zero before using a given section symbol in dynamic relocations.

This patch prevents _bfd_elf_link_renumber_dynsyms from setting any
section symbol dynindx on the first pass.

	PR 22626
	* elflink.c (_bfd_elf_link_renumber_dynsyms): Don't set section
	dynindx when section_sym_count is NULL.
	(bfd_elf_size_dynamic_sections): Pass NULL section_sym_count to
	preliminary _bfd_elf_link_renumber_dynsyms call.