lib/os/prf.c: let the compiler optimize _ldiv5() on 64-bit architectures

The compiler doesn't need help here.

For example, gcc creates this on Aarch64:

_ldiv5:
        ldr     x1, [x0]
        mov     x2, -3689348814741910324
        movk    x2, 0xcccd, lsl 0
        add     x1, x1, 2
        umulh   x1, x1, x2
        lsr     x1, x1, 2
        str     x1, [x0]
        ret

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>

lib/os/prf.c

@@ -123,6 +123,22 @@ static	void _rlrshift(uint64_t *v)
 	*v = (*v & 1) + (*v >> 1);
 }
 
+#ifdef CONFIG_64BIT
+
+static void _ldiv5(uint64_t *v)
+{
+	/*
+	 * Usage in this file wants rounded behavior, not truncation.  So add
+	 * two to get the threshold right.
+	 */
+	*v += 2U;
+
+	/* The compiler can optimize this on its own on 64-bit architectures */
+	*v /= 5U;
+}
+
+#else
+
 /*
  * Tiny integer divide-by-five routine.  The full 64 bit division
  * implementations in libgcc are very large on some architectures, and
@@ -182,6 +198,8 @@ static void _ldiv5(uint64_t *v)
 	*v = result;
 }
 
+#endif
+
 static	char _get_digit(uint64_t *fr, int *digit_count)
 {
 	char rval;