Update LTO plugin interface

[Cary Coutant]

>> For the crtend files we could add a linker option that makes them
>> known as endcaps, and the linker could make sure they get laid out
>> last:
>>
>>    ld ... -lc -lgcc ... --endcap crtend.o crtn.o
>>
>> That puts the special knowledge about those files back in the gcc driver.
>
>  Hmm, yes.  It doesn't work to just pass-through crtn.o, because ...
>
>> Executing on host: /home/davek/gcc/obj-gold2/gcc/xgcc -B/home/davek/gcc/obj-gold2/gcc/ c_lto_20100722-1_0.o  -O0 -flto -flto-partition=none  -fuse-linker-plugin      -o gcc-dg-lto-20100722-1-01    (timeout = 300)
>> gold: /home/davek/gcc/obj-gold2/gcc/crtbegin.o:(.text+0x13): error: undefined reference to '__DTOR_END__'
>> collect2: ld returned 1 exit status
>> compiler exited with status 1
>> output is:
>> gold: /home/davek/gcc/obj-gold2/gcc/crtbegin.o:(.text+0x13): error: undefined reference to '__DTOR_END__'
>> collect2: ld returned 1 exit status
>>
>
> ... it's needed by the first pass of symbol resolution.

Yeah, that's why I originally suggested having the plugin claim it, so
that the plugin could define the symbols when they need to be defined.

-cary