[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..7bf598662af4413bbca2199980ca36abc218d918 100644
--- a/src/base/platform/semaphore.cc
+++ b/src/base/platform/semaphore.cc
@@ -101,6 +101,9 @@ Semaphore::~Semaphore() {
 
 void Semaphore::Signal() {
   int result = sem_post(&native_handle_);
+  // This check may fail with <libc-2.21, which we use on the try bots, if the
+  // semaphore is destroyed while sem_post is still executed. A work around is
+  // to extend the lifetime of the semaphore.
   CHECK_EQ(0, result);
 }