Make TurnSequencer non-x86-safe

Summary:
`TurnSequencer` uses inline assembly, which obviously won't work on ARM.
Guard said assembly to x86{,_64} targets.

Test Plan:
Added an always-false `static_assert` to the `ifdef` block, confirmed it gets
triggered (i.e., we're not removing the `pause` on x86_64).

Reviewed By: ngbronson@fb.com

Subscribers: folly-diffs@, yfeldblum, chalfant

FB internal diff: D2069320

Signature: t1:2069320:1431663264:3b3fb4282cf2b755f9b26c76b608b91ff665e226

folly/detail/TurnSequencer.h

@@ -22,6 +22,7 @@
 #include <unistd.h>
 
 #include <folly/detail/Futex.h>
+#include <folly/Portability.h>
 
 namespace folly {
 
@@ -123,7 +124,9 @@ struct TurnSequencer {
       // the first effectSpinCutoff tries are spins, after that we will
       // record ourself as a waiter and block with futexWait
       if (tries < effectiveSpinCutoff) {
+#if defined(__i386__) || FOLLY_X64
         asm volatile ("pause");
+#endif
         continue;
       }