let FOLLY_KEEP also mark as used, retain

Summary: Attribute `[[gnu::used]]` instructs the compiler to emit what it believes to be an unused definition, and `[[gnu::retain]]` instructs the linker to keep what it believes to be an unreferenced symbol.

Reviewed By: luciang

Differential Revision: D68756388

fbshipit-source-id: 4595a3a4e76582f87f22280489e87733d358c4e5

folly/lang/Keep.h

@@ -56,7 +56,7 @@
 //  This way, the linker will see at least one reference to the kept section,
 //  and so will not throw it out.
 #if __GNUC__ && __linux__
-#define FOLLY_KEEP [[gnu::section(".text.folly.keep")]]
+#define FOLLY_KEEP [[gnu::section(".text.folly.keep"), gnu::used, gnu::retain]]
 #else
 #define FOLLY_KEEP
 #endif