riscv: implement arch_switch()

The move to arch_switch() is a prerequisite for SMP support.

Make it optimal without the need for an ECALL roundtrip on every
context switch. Performance numbers from tests/benchmarks/sched:

Before:
unpend  107 ready  102 switch  188 pend  218 tot  615 (avg  615)

After:
unpend  107 ready  102 switch  170 pend  217 tot  596 (avg  595)

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