[wasm] Move the semaphore for parallel compilation to the wasm module.

src/base/platform/semaphore.cc

Index: src/base/platform/semaphore.cc
diff --git a/src/base/platform/semaphore.cc b/src/base/platform/semaphore.cc
index 7c86f43b323f1b69febd4dbdb75deeb72525feb4..f26c317078940a03cfa3e1960428bb5af77b0c88 100644
--- a/src/base/platform/semaphore.cc
+++ b/src/base/platform/semaphore.cc
@@ -101,6 +101,10 @@ Semaphore::~Semaphore() {
 
 void Semaphore::Signal() {
   int result = sem_post(&native_handle_);
+  // This check may fail with libc-2.19, which we use on the try bots, if the

[Michael Lippautz, 2016/06/22 10:34:46]

nit: Maybe just write that it fails for <libc-2.21?

+  // semaphore is destroyed while sem_post is still executed. A work around is
+  // to extend the lifetime of the semaphore. The problem got fixed in
+  // libc-2.21.
   CHECK_EQ(0, result);
 }