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/runtime_unix.go

@@ -73,6 +73,7 @@ type timespec struct {
 	tv_nsec int64 // unsigned 64-bit integer on all time64 platforms
 }
 
+// Highest address of the stack of the main thread.
 var stackTop uintptr
 
 // Entry point for Go. Initialize all packages and call main.main().