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.

builder/bdwgc.go

@@ -42,7 +42,8 @@ var BoehmGC = Library{
 			// Assertions can be enabled while debugging GC issues.
 			//"-DGC_ASSERTIONS",
 
-			// Threading is not yet supported, so these are disabled.
+			// We use our own way of dealing with threads (that is a bit hacky).
+			// See src/runtime/gc_boehm.go.
 			//"-DGC_THREADS",
 			//"-DTHREAD_LOCAL_ALLOC",