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_tasks.go

@@ -0,0 +1,13 @@
+//go:build scheduler.tasks
+
+package runtime
+
+var systemStack uintptr
+
+// Implementation detail of the internal/task package.
+// It needs to store the system stack pointer somewhere, and needs to know how
+// many cores there are to do so. But it doesn't know the number of cores. Hence
+// why this is implemented in the runtime.
+func systemStackPtr() *uintptr {
+	return &systemStack
+}