Add custom limits.h and timespec.h

These will be needed for added a version of select that is
closer to bsd's select.

rtemsbsd/freebsd/sys/limits.h

@@ -0,0 +1,9 @@
+#include <sys/types.h>
+#include <limits.h>
+
+#define __FD_SETSIZE 1024
+#define FD_SETSIZE __FD_SETSIZE
+
+#define __CHAR_BIT     CHAR_BIT               /* number of bits in a char */
+
+