fma_test: mute a warning about unreachable code on amd64 by restructuring

the #ifdef block to only handle the rest of the logic in the loop in the
#else case.

MFC after:	3 days
Reported by:	Coverity
CID:		1346844
Sponsored by:	Dell EMC Isilon

lib/msun/tests/fma_test.c

@@ -498,11 +498,12 @@ main(void)
 		printf("ok %d # SKIP testcase fails assertion on "
 		    "amd64\n", j);
 		continue;
-#endif
+#else
 		printf("rmode = %d\n", rmodes[i]);
 		fesetround(rmodes[i]);
 		test_infinities();
 		printf("ok %d - fma infinities\n", j);
+#endif
 	}
 
 	fesetround(FE_TONEAREST);