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.

builder/sizes_test.go

@@ -42,7 +42,7 @@ func TestBinarySize(t *testing.T) {
 	// This is a small number of very diverse targets that we want to test.
 	tests := []sizeTest{
 		// microcontrollers
-		{"hifive1b", "examples/echo", 4556, 280, 0, 2268},
+		{"hifive1b", "examples/echo", 4556, 280, 0, 2264},
 		{"microbit", "examples/serial", 2920, 388, 8, 2272},
 		{"wioterminal", "examples/pininterrupt", 7379, 1489, 116, 6912},