ARM: 8155/1: place sigpage at a random offset above stack

ARM: 8155/1: place sigpage at a random offset above stack

The sigpage is currently placed alongside shared libraries etc in the
address space.  Similar to what x86_64 does for its VDSO, place the
sigpage at a randomized offset above the stack so that learning the
base address of the sigpage doesn't help expose where shared libraries
are loaded in the address space (and vice versa).

Signed-off-by: Nathan Lynch <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Signed-off-by: Russell King <[email protected]>