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There are several situations in which the kernel/OS needs to check or
reset the privileged mode stack pointers. Until now the kernel would
have been using hardcoded addresses for this, but that won't work for
SMP setups, where each core will need to have its own unique privileged
mode stacks, or for SVC-mode threading, where each thread is given its
own SVC stack.

To resolve this, store the default stack pointers in a struct in kernel
workspace, and fetch the values from there as needed.

ARMv7 introduces extra, core-local, CP15 registers for storing
thread/process IDs. So on ARMv7+, we'll use the TPIDRPRW register to
hold the pointer to the workspace struct. On startup the kernel will
correctly initialise this register for the primary core; the SMP module
is expected to initialise the register & workspace for the other cores,
as needed. Older CPUs (which are assumed to be single-core) will access
the struct directly via a hardcoded address.