Add thread pool class

Added a thread pool class loosely based on google3 code.

Modified the compiler to have a single thread pool instead of creating new threads in ForAll.

Moved barrier to be in top level directory as it is not GC specific code.

Performance Timings:

Reference:
boot.oat: 14.306596s
time mm oat-target:
real    2m33.748s
user    10m23.190s
sys 5m54.140s

Thread pool:
boot.oat: 13.111049s
time mm oat-target:
real    2m29.372s
user    10m3.130s
sys 5m46.290s

The speed increase is probably just noise.

Change-Id: If3c1280cbaa4c7e4361127d064ac744ea12cdf49
diff --git a/src/thread_list.cc b/src/thread_list.cc
index 4ad25ae..4b2e17f 100644
--- a/src/thread_list.cc
+++ b/src/thread_list.cc
@@ -151,7 +151,7 @@
 }
 #endif
 
-size_t ThreadList::RunCheckpoint(Thread::CheckpointFunction* checkpoint_function) {
+size_t ThreadList::RunCheckpoint(Closure* checkpoint_function) {
   Thread* self = Thread::Current();
   if (kIsDebugBuild) {
     Locks::mutator_lock_->AssertNotHeld(self);