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/gc_stack_raw.go

@@ -1,8 +1,14 @@
-//go:build (gc.conservative || gc.precise || gc.boehm) && !tinygo.wasm && !scheduler.threads
+//go:build (gc.conservative || gc.precise || gc.boehm) && !tinygo.wasm && !scheduler.threads && !scheduler.cores
 
 package runtime
 
-import "internal/task"
+import (
+	"internal/task"
+	"sync/atomic"
+)
+
+// Unused.
+var gcScanState atomic.Uint32
 
 func gcMarkReachable() {
 	markStack()