config/opal_config_asm.m4: trivial output label fix

"OMPI" was a misleading label in the configure output summary of the
atomics choice, and it was weird that there was a whole section just
for Atomics.  Instead, change the output to be "Atomics" in the
Miscellaneous section.  Specifically, the output changes from this:

```
Atomics
-----------------------
OMPI: BUILTIN_C11
```

to this:

```
Miscellaneous
-----------------------
...
Atomics: BUILTIN_C11
```

Signed-off-by: Jeff Squyres <jsquyres@cisco.com>

Author: jsquyres

config/opal_config_asm.m4

@@ -1219,7 +1219,7 @@ int main(int argc, char* argv[])
         [Whether to use builtin atomics])
     AC_SUBST([OPAL_ASSEMBLY_BUILTIN])
 
-    OPAL_SUMMARY_ADD([[Atomics]],[[OMPI]],[],[$opal_cv_asm_builtin])
+    OPAL_SUMMARY_ADD([[Miscellaneous]],[[Atomics]],[],[$opal_cv_asm_builtin])
 
     OPAL_ASM_FIND_FILE