Fix 64-bit clock-related constant value evaluation for AVR compiler

This may be specific for Atmel AVR8 toolchain compiler.
The problem is that despite of being 8-bit architecture
avr-gcc supports uint64_t, but the following code

uint64_t value = 10000 * 1000;

produces a wrong negative value in the final code (tested
both with and without optimization).

The work-around is simple:

uint64_t value = 10000 * 1000L;

The code is a reduced part from sched/signal/sig_timedwait.c where
waitticks64 is calculated using NSEC_PER_TICK. This one is defined
as USEC_PER_TICK * NSEC_PER_USEC which leads to the example above.

include/nuttx/clock.h

@@ -98,16 +98,16 @@
 
 /* Timing constants *********************************************************/
 
-#define NSEC_PER_SEC          1000000000
-#define USEC_PER_SEC             1000000
-#define MSEC_PER_SEC                1000
-#define DSEC_PER_SEC                  10
-#define NSEC_PER_DSEC          100000000
-#define USEC_PER_DSEC             100000
-#define MSEC_PER_DSEC                100
-#define NSEC_PER_MSEC            1000000
-#define USEC_PER_MSEC               1000
-#define NSEC_PER_USEC               1000
+#define NSEC_PER_SEC          1000000000L
+#define USEC_PER_SEC             1000000L
+#define MSEC_PER_SEC                1000L
+#define DSEC_PER_SEC                  10L
+#define NSEC_PER_DSEC          100000000L
+#define USEC_PER_DSEC             100000L
+#define MSEC_PER_DSEC                100L
+#define NSEC_PER_MSEC            1000000L
+#define USEC_PER_MSEC               1000L
+#define NSEC_PER_USEC               1000L
 
 /* If CONFIG_SCHED_TICKLESS is not defined, then the interrupt interval of
  * the system timer is given by USEC_PER_TICK.  This is the expected number