bfd/elf: Don't read non-existing secondary relocs

bfd/elf: Don't read non-existing secondary relocs

Without this we unconditionally try to slurp in secondary
relocs for each input section, leading to quadratic behaviour
even for strip(1).  On write-out we already used a flag to avoid
this.

So track existence of secondary relocs on read-in as well and
only slurp in when needed.  This still doesn't implement a proper
list of secondary reloc sections, and still would exhibit quadratic
behaviour if most input sections have a secondary reloc section.
But at least on normal input this avoids any slowdown from trying
to handle secondary relocation sections.

bfd/
	* elf.c (bfd_section_from_shdr): Set has_secondary_relocs flag.
	(_bfd_elf_slurp_secondary_reloc_section): Use it for early-out.