csu: Change start code license to have link exception

The start code can get linked into dynamic linked executables where
LGPL would require shipping the source or linkable binaries when the
executable is distributed.

On some targets the license exception was missing in start.S (which
is compiled into crt1.o and Scrt1.o which may end up linked into PDE
and PIE binaries).

I did not review what other code may end up in executables, just
fixed the start.S license inconsistency across targets.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>