powerpc: Relocate stinfo->main

start_addresses in sysdeps/powerpc/powerpc64/start.S is historical
baggage that should disappear.  Until someone does that, relocating
stinfo->main by hand is one solution to the fact that the field may be
unrelocated at the time it is accessed.  This is similar to what is
done for dynamic tags via the D_PTR macro.  stinfo->init and
stinfo->fini are zero in both powerpc64/start.S and powerpc32/start.S,
so make it a little more obvious they are unused by passing NULLs to
LIBC_START_MAIN.  The makefile change is needed to pick up
elf/dl-static-tls.h from dl-machine.h.

Reviewed-by: Tulio Magno Quites Machado Filho <tuliom@linux.ibm.com>