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>