all: add support for multicore scheduler

This commit adds support for a scheduler that runs a scheduler on all
available cores. It is meant to be used on baremetal systems with a
fixed number of cores, such as the RP2040.

The initial implementation adds support for multicore scheduling to the
riscv-qemu target as a convenient testing target. This means that this
new multicore scheduler is tested in CI, including a bunch of standard
library tests (`make tinygo-test-baremetal`). This should ensure the new
scheduler is reasonably well tested before trying to use it on
harder-to-debug targets like the RP2040.

src/runtime/scheduler_none.go

@@ -2,7 +2,10 @@
 
 package runtime
 
-import "internal/task"
+import (
+	"internal/task"
+	"runtime/interrupt"
+)
 
 const hasScheduler = false
 
@@ -73,3 +76,19 @@ func scheduler(returnAtDeadlock bool) {
 	// this code should be unreachable.
 	runtimePanic("unreachable: scheduler must not be called with the 'none' scheduler")
 }
+
+func lockAtomics() interrupt.State {
+	return interrupt.Disable()
+}
+
+func unlockAtomics(mask interrupt.State) {
+	interrupt.Restore(mask)
+}
+
+func printlock() {
+	// nothing to do
+}
+
+func printunlock() {
+	// nothing to do
+}