Remove global init of ThreadLocal in Random

Summary:
Remove global initialization of ThreadLocal in Random
See https://gcc.gnu.org/onlinedocs/gcc-4.8.3/gcc/Thread-Local.html

Static initializers and thread locals don't mix well, and we're seeing
some crashes which we think might be related to this.

Test Plan: Unit tests

Reviewed By: seanc@fb.com

Subscribers: trunkagent, seanc, njormrod

FB internal diff: D1617455

folly/Random.cpp

@@ -90,11 +90,11 @@ void BufferedRandomDevice::getSlow(unsigned char* data, size_t size) {
   ptr_ += size;
 }
 
-ThreadLocal<BufferedRandomDevice> bufferedRandomDevice;
 
 }  // namespace
 
 void Random::secureRandom(void* data, size_t size) {
+  static ThreadLocal<BufferedRandomDevice> bufferedRandomDevice;
   bufferedRandomDevice->get(data, size);
 }