[GOLD] Set non-exec stack for ppc64

[GOLD] Set non-exec stack for ppc64

gcc doesn't emit stack notes for ELFv1, since ELFv1 never needs an
executable stack.  Note that ELFv1 is usually big-endian and ELFv2
little-endian, but the ABI is really orthogonal to endiannes.

	* powerpc.cc (Target_powerpc<64,*>::powerpc_info): Set
	is_default_stack_executable false.