cpukit/score/src/kern_ntptime.c: Negative Shift Value Warning

The code in question was shifting a signed integer. This was
caught by the -Wshift-negative-value GCC warning.

Closes #5339.

cpukit/score/src/kern_ntptime.c

@@ -116,6 +116,15 @@ typedef int64_t l_fp;
 #define L_MPY(v, a)	((v) *= (a))
 #define L_CLR(v)	((v) = 0)
 #define L_ISNEG(v)	((v) < 0)
+#ifdef __rtems__
+#define L_LINT(v, a) \
+	do { \
+		if ((a) < 0) \
+			((v) = -((uint64_t)(-(a)) << 32)); \
+		else \
+			((v) = (uint64_t)(a) << 32); \
+	} while (0)
+#else
 #define L_LINT(v, a) \
 	do { \
 		if ((a) < 0) \
@@ -123,6 +132,7 @@ typedef int64_t l_fp;
 		else \
 			((v) = (int64_t)(a) << 32); \
 	} while (0)
+#endif /* __rtems__ */
 #define L_GINT(v)	((v) < 0 ? -(-(v) >> 32) : (v) >> 32)
 
 /*