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/internal/task/futex-threads.go

@@ -0,0 +1,7 @@
+//go:build scheduler.threads
+
+package task
+
+import "internal/futex"
+
+type Futex = futex.Futex