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_blocks.go

@@ -467,8 +467,7 @@ func runGC() (freeBytes uintptr) {
 	}
 
 	// Mark phase: mark all reachable objects, recursively.
-	markStack()
-	findGlobals(markRoots)
+	gcMarkReachable()
 
 	if baremetal && hasScheduler {
 		// Channel operations in interrupts may move task pointers around while we are marking.
@@ -502,6 +501,10 @@ func runGC() (freeBytes uintptr) {
 		finishMark()
 	}
 
+	// If we're using threads, resume all other threads before starting the
+	// sweep.
+	gcResumeWorld()
+
 	// Sweep phase: free all non-marked objects and unmark marked objects for
 	// the next collection cycle.
 	freeBytes = sweep()