Conditionally decrease minimum gcc version to 9

Summary:
As discussed in this post: https://fb.workplace.com/groups/560979627394613/permalink/3004000963092455/

Introduce FOLLY_CONFIG_TEMPORARY_DOWNGRADE_GCC that allows to downgrade temporarily to gcc 9.

The symbol is used only with BrightSign toolchain that runs on gcc 9.5. More details about BrightSign SDK are in this doc: https://docs.google.com/document/d/1BdDOu_qx92WuezZYJ1EUoJyntzmgZ1n8D5Ecq43eCAo/edit?usp=sharing

Reviewed By: skrueger

Differential Revision: D61489249

fbshipit-source-id: 959c458584f37265ec9b0f593cd1e101070a1d35

folly/Portability.h

@@ -35,8 +35,12 @@
 static_assert(FOLLY_CPLUSPLUS >= 201703L, "__cplusplus >= 201703L");
 
 #if defined(__GNUC__) && !defined(__clang__)
+#if defined(FOLLY_CONFIG_TEMPORARY_DOWNGRADE_GCC)
+static_assert(__GNUC__ >= 9, "__GNUC__ >= 9");
+#else
 static_assert(__GNUC__ >= 10, "__GNUC__ >= 10");
 #endif
+#endif
 
 #if defined(_MSC_VER)
 static_assert(_MSC_VER >= 1920);