use cc provided xlog2 instead of system log2.

(on plan 9 cc calls it log2, but that conflicts here.)
the difference is that xlog2 returns -1 on non powers of 2.
8c was rewriting /10 into /8.

R=ken
OCL=29968
CL=29968

src/cmd/8c/cgen.c

@@ -346,7 +346,7 @@ cgen(Node *n, Node *nn)
 				c = r->vconst;
 				if(c < 0)
 					c = -c;
-				v = log2(c);
+				v = xlog2(c);
 				if(v < 0)
 					break;
 				/* fall thru */
@@ -609,7 +609,7 @@ cgen(Node *n, Node *nn)
 				c = r->vconst;
 				if(c < 0)
 					c = -c;
-				v = log2(c);
+				v = xlog2(c);
 				if(v < 0)
 					break;
 				/* fall thru */