core: Fixed build constraints

Multiple constraints on the same line are combined with logical OR,
while combining multiple negated constraints needs to be done with
logical AND (each constraint on a separate line).

core/fmt/fmt_os.odin

@@ -1,4 +1,5 @@
-//+build !freestanding !js
+//+build !freestanding
+//+build !js
 package fmt
 
 import "core:runtime"