Move uinteger_pow gdb/valarith.c to gdb/utils.c and make it public

This is a generic function which I would like to use in a followup
patch adding support for fixed-point types. So this commit moves it
out of valarith.c into util.c, and makes it non-static.

gdb/ChangeLog:

        * utils.h (uinteger_pow): Add declaration.
        * utils.c (uinteger_pow): Moved here (without changes)...
        * valarith.c (uinteger_pow): ... from here.

gdb/valarith.c

@@ -819,37 +819,6 @@ integer_pow (LONGEST v1, LONGEST v2)
     }
 }
 
-/* Integer exponentiation: V1**V2, where both arguments are
-   integers.  Requires V1 != 0 if V2 < 0.  Returns 1 for 0 ** 0.  */
-
-static ULONGEST
-uinteger_pow (ULONGEST v1, LONGEST v2)
-{
-  if (v2 < 0)
-    {
-      if (v1 == 0)
-	error (_("Attempt to raise 0 to negative power."));
-      else
-	return 0;
-    }
-  else 
-    {
-      /* The Russian Peasant's Algorithm.  */
-      ULONGEST v;
-      
-      v = 1;
-      for (;;)
-	{
-	  if (v2 & 1L) 
-	    v *= v1;
-	  v2 >>= 1;
-	  if (v2 == 0)
-	    return v;
-	  v1 *= v1;
-	}
-    }
-}
-
 /* Obtain argument values for binary operation, converting from
    other types if one of them is not floating point.  */
 static void