Add a couple of nil function implementations.

pkg/runtime/goroutine.c

@@ -26,7 +26,7 @@ SOFTWARE.
 #include <pthread.h>
 #include <stdlib.h>
 
-void go(struct Func) __asm__("runtime.go");
+void go(struct Func) LLGO_ASM_EXPORT("runtime.go");
 
 static void* call_gofunction(void *arg)
 {

pkg/runtime/proc.ll

@@ -0,0 +1,14 @@
+; Copyright 2013 The llgo Authors.
+; Use of this source code is governed by an MIT-style
+; license that can be found in the LICENSE file.
+
+define void @syscall.runtime_BeforeFork() {
+	; TODO
+	ret void
+}
+
+define void @syscall.runtime_AfterFork() {
+	; TODO
+	ret void
+}
+

pkg/runtime/sem.ll

@@ -12,3 +12,17 @@ define void @sync.runtime_Semrelease(i32*) {
 	ret void
 }
 
+define void @sync.runtime_Syncsemacquire(i32*) {
+	; TODO
+	ret void
+}
+
+define void @sync.runtime_Syncsemrelease(i32*) {
+	; TODO
+	ret void
+}
+
+define void @sync.runtime_Syncsemcheck(i32*) {
+	; TODO
+	ret void
+}