leon: Add implementation of nearbyint.

leon-common/nearbyint.c

+#include <stdint.h>
+#include <math.h>
+
+static const double toint = 4503599627370496.0; // 2**52
+
+double nearbyint(double x) {
+    union { double f; uint64_t i; } u = {x};
+    int e = u.i >> 52 & 0x7ff;
+    int s = u.i >> 63;
+
+    if (e >= 0x3ff + 52) {
+        // Large enough that it doesn't have a fraction
+    } else if (s) {
+        x -= toint;
+        x += toint;
+        if (x == 0) {
+            x = -0.0;
+        }
+    } else {
+        x += toint;
+        x -= toint;
+    }
+
+    return x;
+}