arch: arm: cortex_r: Do not use user stack in svc/isr modes

The user thread cannot be trusted so do not use the stack pointer it
passes in.  Use the thread's privilege stack when in privileged modes to
make sure a user thread does not trick the svc/isr handlers into writing
to memory it should not.

Signed-off-by: Bradley Bolen <bbolen@lexmark.com>