implement llabs in case the compiler doesn't inline it

src/lib/arithmetic.c

 #include <stdint.h>
+#include <stdlib.h>
 
 /* On x86, division of one 64-bit integer by another cannot be
    done with a single instruction or a short sequence.  Thus, GCC
@@ -152,6 +153,12 @@ smod64 (int64_t n, int64_t d)
 {
   return n - d * sdiv64 (n, d);
 }
+
+long long
+llabs(long long j)
+{
+    return j >= 0 ? j : -j;
+}
 
 /* These are the routines that GCC calls. */