riscv/kernel: update __show_regs() to print shadow stack register

riscv/kernel: update __show_regs() to print shadow stack register

Update __show_regs() to print the captured shadow stack pointer.  On
tasks where shadow stack is disabled, simply print 0.

Signed-off-by: Deepak Gupta <debug@rivosinc.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Tested-by: Andreas Korb <andreas.korb@aisec.fraunhofer.de> # QEMU, custom CVA6
Tested-by: Valentin Haudiquet <valentin.haudiquet@canonical.com>
Link: https://patch.msgid.link/20251112-v5_user_cfi_series-v23-18-b55691eacf4f@rivosinc.com
[pjw@kernel.org: cleaned up patch description]
Signed-off-by: Paul Walmsley <pjw@kernel.org>