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/print.go

@@ -1,7 +1,6 @@
 package runtime
 
 import (
-	"internal/task"
 	"unsafe"
 )
 
@@ -9,18 +8,6 @@ type stringer interface {
 	String() string
 }
 
-// Lock to make sure print calls do not interleave.
-// This is a no-op lock on systems that do not have parallelism.
-var printLock task.PMutex
-
-func printlock() {
-	printLock.Lock()
-}
-
-func printunlock() {
-	printLock.Unlock()
-}
-
 //go:nobounds
 func printstring(s string) {
 	for i := 0; i < len(s); i++ {