aws-c-common: emit unwind information for RISC-V

Pass -fasynchronous-unwind-tables in CFLAGS for RISC-V.

This fixes functionality which depends on backtrace() and enables
running the test suite without error.

pkgs/development/libraries/aws-c-common/default.nix

@@ -21,6 +21,8 @@ stdenv.mkDerivation rec {
   cmakeFlags = [
     "-DBUILD_SHARED_LIBS=ON"
     "-DCMAKE_SKIP_BUILD_RPATH=OFF" # for tests
+  ] ++ lib.optionals stdenv.hostPlatform.isRiscV [
+    "-DCMAKE_C_FLAGS=-fasynchronous-unwind-tables"
   ];
 
   # aws-c-common misuses cmake modules, so we need