runtime: map every goroutine to a new OS thread

This is not a scheduler in the runtime, instead every goroutine is
mapped to a single OS thread - meaning 1:1 scheduling.

While this may not perform well (or at all) for large numbers of
threads, it greatly simplifies many things in the runtime. For example,
blocking syscalls can be called directly instead of having to use epoll
or similar. Also, we don't need to do anything special to call C code -
the default stack is all we need.

src/runtime/gc_stack_raw.go

@@ -1,9 +1,14 @@
-//go:build (gc.conservative || gc.precise || gc.boehm) && !tinygo.wasm
+//go:build (gc.conservative || gc.precise || gc.boehm) && !tinygo.wasm && !scheduler.threads
 
 package runtime
 
 import "internal/task"
 
+func gcMarkReachable() {
+	markStack()
+	findGlobals(markRoots)
+}
+
 // markStack marks all root pointers found on the stack.
 //
 // This implementation is conservative and relies on the stack top (provided by
@@ -36,3 +41,7 @@ func scanstack(sp uintptr) {
 		markCurrentGoroutineStack(sp)
 	}
 }
+
+func gcResumeWorld() {
+	// Nothing to do here (single threaded).
+}