riscv: Fix SMP when shadow call stacks are enabled

riscv: Fix SMP when shadow call stacks are enabled

This fixes two bugs in SCS initialization for secondary CPUs. First,
the SCS was not initialized at all in the spinwait boot path. Second,
the code for the SBI HSM path attempted to initialize the SCS before
enabling the MMU. However, that involves dereferencing the thread
pointer, which requires the MMU to be enabled.

Fix both issues by setting up the SCS in the common secondary entry
path, after enabling the MMU.

Fixes: d1584d791a29 ("riscv: Implement Shadow Call Stack")
Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Link: https://lore.kernel.org/r/20231121211958.3158576-1-samuel.holland@sifive.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>