Incorporate review comments

Author: kleisauke

src/library_pthread.js

@@ -409,6 +409,10 @@ var LibraryPThread = {
 
   __emscripten_thread_cleanup: function(thread) {
     // Called when a thread needs to be cleaned up so it can be reused.
+    // A thread is considered reusable when it either returns from its
+    // entry point, calls pthread_exit, or acts upon a cancellation.
+    // Detached threads are responsible for calling this themselves,
+    // otherwise pthread_join is responsible for calling this.
     if (!ENVIRONMENT_IS_PTHREAD) cleanupThread(thread);
     else postMessage({ 'cmd': 'cleanupThread', 'thread': thread });
   },

system/lib/libc/musl/src/thread/pthread_cancel.c

@@ -3,6 +3,7 @@
 #include "pthread_impl.h"
 #include "syscall.h"
 
+#ifndef __EMSCRIPTEN__
 hidden long __cancel(), __syscall_cp_asm(), __syscall_cp_c();
 
 long __cancel()
@@ -14,7 +15,6 @@ long __cancel()
 	return -ECANCELED;
 }
 
-#ifndef __EMSCRIPTEN__
 long __syscall_cp_asm(volatile void *, syscall_arg_t,
                       syscall_arg_t, syscall_arg_t, syscall_arg_t,
                       syscall_arg_t, syscall_arg_t, syscall_arg_t);
@@ -67,7 +67,6 @@ static void cancel_handler(int sig, siginfo_t *si, void *ctx)
 
 	__syscall(SYS_tkill, self->tid, SIGCANCEL);
 }
-#endif
 
 void __testcancel()
 {
@@ -76,7 +75,6 @@ void __testcancel()
 		__cancel();
 }
 
-#ifndef __EMSCRIPTEN__
 static void init_cancellation()
 {
 	struct sigaction sa = {