[Xtensa] Add a no-op -mlongcalls option for better compatibility

Many projects targeting Xtensa architecture use GCC-specific
-mlongcalls option. The current behavior of LLVM for Xtensa is
equivalent to this option being set, so accept this option without
changing the behavior.

Author: andreisfr

clang/include/clang/Driver/Options.td

@@ -4263,6 +4263,7 @@ def mfix_esp32_psram_cache_issue : Flag<["-"], "mfix-esp32-psram-cache-issue">,
 def mfix_esp32_psram_cache_strategy_EQ : Joined<["-"], "mfix-esp32-psram-cache-strategy=">, Group<m_xtensa_Features_Group>,
     HelpText<" Psram cache fix strategies : memw, nops">,
     Values<"memw, nops">;
+def mlongcalls : Flag<["-"], "mlongcalls">, Group<m_xtensa_Features_Group>;
 
 // These are legacy user-facing driver-level option spellings. They are always
 // aliases for options that are spelled using the more common Unix / GNU flag