Randomise the lower bits of the stack address. More randomisation is good for
security but the scatter can also help with SMT threads that share an L1. A
quick test case shows this working:

int main()
{
	int sp;
	printf("%x\n", (unsigned long)&sp & 4095);
}

before:
80
80
80
80
80

after:
610
490
300
6b0
d80
Signed-off-by: NAnton Blanchard <anton@samba.org>
Signed-off-by: NBenjamin Herrenschmidt <benh@kernel.crashing.org>