riscv: isr.S: dedicate a register to &current_cpu

Stop using &_kernel as this is not SMP friendly. Let's use s0 (after
preserving its content) to hold &current_cpu instead so it won't have
to be reloaded 	each time it is needed. This will be even more relevant
when SMP support is added.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>