libnetbsd: add math.h and sys/time.h

- `math.h`: `isinff(..)`
- `sys/time.h`: `timespec*(x)`

These two headers are used by tests in newer snapshots of
`contrib/netbsd-tests`.

MFC after:	1 week

lib/libnetbsd/math.h

@@ -0,0 +1,14 @@
+/*-
+ * SPDX-License-Identifier: BSD-2-Clause
+ *
+ * Copyright (c) 2026 Enji Cooper
+ */
+
+#ifndef	__LIBNETBSD_MATH_H__
+#define	__LIBNETBSD_MATH_H__
+
+#include_next <math.h>
+
+#define	isinff(x)	__isinff(x)
+
+#endif

lib/libnetbsd/sys/time.h

@@ -0,0 +1,35 @@
+/*-
+ * SPDX-License-Identifier: BSD-2-Clause
+ *
+ * Copyright (c) 2026 Enji Cooper
+ */
+
+#ifndef _LIBNETBSD_SYS_TIME_H_
+#define _LIBNETBSD_SYS_TIME_H_
+
+#include_next <sys/time.h>
+
+#define timercmp(tvp, uvp, cmp)						\
+	(((tvp)->tv_sec == (uvp)->tv_sec) ?				\
+	    ((tvp)->tv_usec cmp (uvp)->tv_usec) :			\
+	    ((tvp)->tv_sec cmp (uvp)->tv_sec))
+#define timespecadd(tsp, usp, vsp)					\
+	do {								\
+		(vsp)->tv_sec = (tsp)->tv_sec + (usp)->tv_sec;		\
+		(vsp)->tv_nsec = (tsp)->tv_nsec + (usp)->tv_nsec;	\
+		if ((vsp)->tv_nsec >= 1000000000L) {			\
+			(vsp)->tv_sec++;				\
+			(vsp)->tv_nsec -= 1000000000L;			\
+		}							\
+	} while (0)
+#define timespecsub(tsp, usp, vsp)					\
+	do {								\
+		(vsp)->tv_sec = (tsp)->tv_sec - (usp)->tv_sec;		\
+		(vsp)->tv_nsec = (tsp)->tv_nsec - (usp)->tv_nsec;	\
+		if ((vsp)->tv_nsec < 0) {				\
+			(vsp)->tv_sec--;				\
+			(vsp)->tv_nsec += 1000000000L;			\
+		}							\
+	} while (0)
+
+#endif