[GOLD] PowerPC64 huge branch dynamic relocs

[GOLD] PowerPC64 huge branch dynamic relocs

PowerPC64 gold and ld.bfd implement an indirect branch trampoline,
used when the destination of a branch exceeds a bounce through another
"b" instruction.  When generating PIEs or shared libraries, the
addresses need dynamic relocations.  This was implemented in gold
using a dedicated relocation section, but this means the relative
relocations for these addresses are not sorted properly with other
dynamic relative relocations: gold doesn't support merging relocation
sections, then sorting.  Instead we need to use a single .rela.dyn
section.

This is done by increasing the size of rela_dyn_ during do_relax to
account for needed dynamic relocations, delaying adding the actual
relocations until the end of relaxation once the layout has
stabilised.

	* powerpc.cc (Target_powerpc): Add rela_dyn_size_;
	(update_current_size): New function.
	(Target_powerpc::do_relax): Capture the size of rela_dyn_ at
	the start of relaxation.  Artifically increase its size during
	relaxation to account for needed indirect branches, and add
	those relocations at the end.
	(Output_data_brlt_powerpc::rel_, reset_brlt_sizes),
	(finalize_brlt_sizes, add_reloc, set_current_size): Delete.
	(Target_powerpc::make_brlt_section): Don't make reloc section.