[GOLD] PowerPC notoc eh_frame

When generating notoc call and branch stubs without the benefit of
pc-relative insns, the stubs need to use LR to access the run time PC.
All LR changes must be described in .eh_frame if we're to support
unwinding through asynchronous exceptions.  That's what this patch
does.

The patch has gone through way too many iterations.  At first I
attempted to add multiple FDEs, one for each stub.  That ran into
difficulties with do_plt_fde_location which is only capable of setting
the address of a single FDE per Output_data section, and with removing
any FDEs added on a previous do_relax pass.  Removing FDEs (git commit
be897fb774) went overboard in matching the FDE contents.  That means
either stashing the contents created for add_eh_frame_for_plt to use
when calling remove_eh_frame_for_plt, or recreating contents on the
fly (*) just to remove FDEs.  In fact, FDE content matching is quite
unnecesary.  FDEs added by a previous do_relax pass are those with
u_.from_linker.post_map set.  So they can easily be recognised just by
looking at that flag.  This patch keeps that part of the multiple FDE
changes.

In the end I went for just one FDE per stub group to describe the call
stubs.  That's reasonably efficient for the common case of only
needing to describe the __tls_get_addr_opt call stub.  We don't expect
to be making many calls using notoc stubs without pc-relative insns.

*) Which has it's own set of problems.  The contents must be recreated
using the old stub layout, but .eh_frame size can affect stub
requirements so you need to temporarily keep the old .eh_frame size
when creating new stubs, then reset .eh_frame size before adding new
FDEs.

	* ehframe.cc (Fde::operator==): Delete.
	(Cie::remove_fde): Delete.
	(Eh_frame::remove_ehframe_for_plt): Delete fde_data and fde_length
	parameters.  Remove all post-map plt FDEs.
	* ehframe.h (Fde:post_map): Make const, add variant to compare plt.
	(Fde::operator==): Delete.
	(Cie::remove_fde): Implement here.
	(Cie::last_fde): New accessor.
	(Eh_frame::remove_ehframe_for_plt): Update prototype.
	* layout.cc (Layout::remove_eh_frame_for_plt): Delete fde_data and
	fde_length parameters.
	* layout.h (Layout::remove_eh_frame_for_plt): Update prototype.
	* powerpc.cc (Stub_table::tls_get_addr_opt_bctrl_): Delete.
	(Stub_table::plt_fde_len_, plt_fde_, init_plt_fde): Delete.
	(Stub_table::add_plt_call_entry): Don't set tls_get_addr_opt_bctrl_.
	(eh_advance): New function.
	(stub_sort): New function.
	(Stub_table::add_eh_frame): Emit eh_frame for notoc plt calls and
	branches as well as __tls_get_addr_opt plt call stub.
	(Stub_table::remove_eh_frame): Update to suit.

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