Enable assert() for release builds

Our assert() statements are pretty cheap. We want to enable them
even for release builds.

By default, cmake passes -DNDEBUG to disable assert() for release
builds. So, in this patch, we undefine it to re-enable assertions.

mold.h

@@ -4,6 +4,8 @@
 #define _GNU_SOURCE
 #endif
 
+#undef NDEBUG
+
 #include "elf.h"
 
 #include <atomic>