[build] Move Solaris 2 startup files to toplevel libgcc, revised

[Eric Botcazou]

> * Non-Solaris SPARC changes:
>
>   After I had moved sparc/sol2-c[in].asm to libgcc, I noticed that
>   despite the name a few non-Solaris targets uses those files, too:
>
>   sparc-*-elf*, sparc-*-rtems*, sparc64-*-elf*, sparc64-*-rtems*

Yes, I tried to untangle that, but the RTEMS folks complained so I had to 
backpedal.  Note that this is also the case on the i386 side.

-- 
Eric Botcazou