BuildSystem: Move all userpace libraries under the userspace directory

As the number of libraries is increasing, root directory starts to
expand. This adds better organization for libraries

userspace/libraries/LibC/include/tgmath.h

@@ -0,0 +1,17 @@
+#ifndef _TGMATH_H
+#define _TGMATH_H 1
+
+// https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/tgmath.h.html
+
+#include <sys/cdefs.h>
+
+__BEGIN_DECLS
+
+#include <math.h>
+#include <complex.h>
+
+// FIXME
+
+__END_DECLS
+
+#endif