mjit.c: suppress warnings by -Wunused-value

on MinGW with __atomic_exchange_n(). It returns the previous pointer value but
obviously it's not needed in MJIT's usage.

mjit.c: In function 'compact_all_jit_code':
ruby_atomic.h:11:36: warning: value computed is not used [-Wunused-value]
 # define ATOMIC_EXCHANGE(var, val) __atomic_exchange_n(&(var), (val), __ATOMIC_SEQ_CST)
                                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64160 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

mjit.c

@@ -140,7 +140,7 @@ typedef intptr_t pid_t;
 #endif
 
 /* Atomically set function pointer if possible. */
-#define MJIT_ATOMIC_SET(var, val) ATOMIC_PTR_EXCHANGE(var, val)
+#define MJIT_ATOMIC_SET(var, val) (void)ATOMIC_PTR_EXCHANGE(var, val)
 
 /* A copy of MJIT portion of MRI options since MJIT initialization.  We
    need them as MJIT threads still can work when the most MRI data were