excise AccessSpreader extern-template-struct

Summary:
There appear always to be problems with `extern template struct`. In this case, some compilers either not emitting or dropping thread-local wrapper functions.

```
stderr: Undefined symbols for architecture x86_64:
  "thread-local wrapper routine for folly::AccessSpreader<std::__1::atomic>::cpuCache", referenced from:
      folly::AccessSpreader<std::__1::atomic>::cachedCurrent(unsigned long) in CacheLocality.cpp.o
  "thread-local wrapper routine for folly::SequentialThreadId<std::__1::atomic>::currentId", referenced from:
      folly::SequentialThreadId<std::__1::atomic>::get() in CacheLocality.cpp.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
```

Reviewed By: Mizuchi

Differential Revision: D27579816

fbshipit-source-id: b539816fb1f62d9e50e22d6cbc3c91a0d82ac629

folly/concurrency/CacheLocality.cpp

@@ -318,11 +318,6 @@ Getcpu::Func Getcpu::resolveVdsoFunc() {
 #endif
 }
 
-#ifdef FOLLY_CL_USE_FOLLY_TLS
-/////////////// SequentialThreadId
-template struct SequentialThreadId<std::atomic>;
-#endif
-
 namespace detail {
 
 int AccessSpreaderBase::degenerateGetcpu(unsigned* cpu, unsigned* node, void*) {
@@ -370,9 +365,6 @@ bool AccessSpreaderBase::initialize(
 
 } // namespace detail
 
-/////////////// AccessSpreader
-template struct AccessSpreader<std::atomic>;
-
 SimpleAllocator::SimpleAllocator(size_t allocSize, size_t sz)
     : allocSize_{allocSize}, sz_(sz) {}
 

folly/concurrency/CacheLocality.h

@@ -170,10 +170,6 @@ Atom<unsigned> SequentialThreadId<Atom>::prevId(0);
 
 template <template <typename> class Atom>
 FOLLY_TLS unsigned SequentialThreadId<Atom>::currentId(0);
-
-// Suppress this instantiation in other translation units. It is
-// instantiated in CacheLocality.cpp
-extern template struct SequentialThreadId<std::atomic>;
 #endif
 
 struct HashingThreadId {
@@ -395,10 +391,6 @@ FOLLY_TLS
 template <template <typename> class Atom>
 bool AccessSpreader<Atom>::initialized = AccessSpreader<Atom>::initialize();
 
-// Suppress this instantiation in other translation units. It is
-// instantiated in CacheLocality.cpp
-extern template struct AccessSpreader<std::atomic>;
-
 /**
  * A simple freelist allocator.  Allocates things of size sz, from
  * slabs of size allocSize.  Takes a lock on each