riscv: fix integer wraparound in memcpy

This patch fixes a bug in RISC-V's memcpy implementation where an
integer wraparound occurs when src + size < 8 * sizeof(long), causing
the word-sized copy loop to be incorrectly entered.

Signed-off-by: Chih-Mao Chen <cmchen@andestech.com>

newlib/libc/machine/riscv/memcpy.c

@@ -51,9 +51,9 @@ small:
   const long *lb = (const long *)b;
   long *lend = (long *)((uintptr_t)end & ~msk);
 
-  if (unlikely (la < (lend - 8)))
+  if (unlikely (lend - la > 8))
     {
-      while (la < (lend - 8))
+      while (lend - la > 8)
 	{
 	  long b0 = *lb++;
 	  long b1 = *lb++;