bpo-40878: xlc cannot handle C99 extern inline. (GH-21887)

This applies to the default "extc99" mode.  Python does not compile with "stdc99".

Modules/_decimal/libmpdec/mpdecimal.c

@@ -64,7 +64,7 @@
 
 #if defined(_MSC_VER)
   #define ALWAYS_INLINE __forceinline
-#elif defined(LEGACY_COMPILER)
+#elif defined(__IBMC__) || defined(LEGACY_COMPILER)
   #define ALWAYS_INLINE
   #undef inline
   #define inline