What to do when sh_addralign isn't a power of two

BFD generally doesn't handle anything but a power of two section
alignment, and ELF sh_addralign is required to be an integral power of
two (or zero) by the ELF spec.  Of course this is ignored by fuzzers,
and because bfd_log2 rounds up, we can end up with alignment_power
being 32 on a 32-bit object or 64 on a 64-bit object.  That then
triggers ubsan warnings in places like bfd_update_compression_header
where we want to convert from alignment_power back to an alignment.
I suppose we could reject object files that have non-compliant
sh_addralign, but I think it's also reasonable to use the greatest
power of two divisor of sh_addralign, ie. the rightmost 1 bit.

	* elf.c (_bfd_elf_make_section_from_shdr): Use greatest power
	of two divisor of sh_addralign.
	(_bfd_elf_assign_file_position_for_section): Likewise.
	(assign_file_positions_for_non_load_sections): Likewise.

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