Only try to use the membarrier on Linux

Summary: It doesn't exist anywhere else, and `syscall` isn't even defined on Windows, so this completely failed to compile before.

Reviewed By: yfeldblum

Differential Revision: D3623382

fbshipit-source-id: 302093ddc96d6465e0dcc1883747229a7fb25a84

folly/portability/SysMembarrier.cpp

@@ -21,7 +21,8 @@
 #include <folly/Portability.h>
 #include <folly/portability/Unistd.h>
 
-#if !defined(__NR_membarrier) && FOLLY_X64 && !FOLLY_MOBILE
+#if !defined(__NR_membarrier) && FOLLY_X64 && !FOLLY_MOBILE && \
+    defined(__linux__)
 #define __NR_membarrier 324
 #define MEMBARRIER_CMD_QUERY 0
 #define MEMBARRIER_CMD_SHARED 1