[GOLD] PowerPC recreate eh_frame for stubs on each relax pass

[Cary Coutant]

> There is a very small but non-zero probability that a stub group
> contains stubs on one relax pass, but does not on the next.  In that
> case we would get an FDE covering a zero length address range.
> (Actually, it's even worse.  Alignment padding for stubs can mean the
> address for the non-existent stubs is past the end of the original
> section to which stubs are attached, and due to the way
> do_plt_fde_location calculates the length we can get a negative
> length.)  Fixing this properly requires removing the FDE.
>
> Also, I have been implementing the __tls_get_addr_opt support for
> gold, and that stub needs something other than the default FDE.  The
> necessary FDE will depend on the offset to the __tls_get_addr_opt
> stub, which of course can change during relaxation.  That means at the
> very least, rewriting the FDE on each pass, possibly changing the FDE
> size.  I think that is better done by completely recreating PLT
> eh_frame FDEs.
>
> OK?
>
>         * ehframe.cc (Fde::operator==): New.
>         (Cie::remove_fde, Eh_frame::remove_ehframe_for_plt): New.
>         * ehframe.h (Fde::operator==): Declare.
>         (Cie::remove_fde, Eh_frame::remove_ehframe_for_plt): Likewise.
>         * layout.cc (Layout::remove_eh_frame_for_plt): New.
>         * layout.h (Layout::remove_eh_frame_for_plt): Declare.
>         * powerpc.cc (Target_powerpc::do_relax): Remove old eh_frame FDEs.
>         (Stub_table::add_eh_frame): Delete eh_frame_added_ condition.
>         Don't add eh_frame for empty stub section.
>         (Stub_table::remove_eh_frame): New.

This is OK. Thanks!

-cary