Avoid hardcoding "gcc" in the DTrace tests.

At least one test doesn't work yet without gcc, however gcc is
not always available in base. Using the environment compiler
is more trustable and will also work with an external compiler.

Reviewed by:	markj
MFC after:	3 days

tools/test/dtrace/Makefile

@@ -337,7 +337,7 @@ listprgs	:
 
 .for _f in ${ALL_TESTCFILES}
 ${_f:T:S/c$/exe/} : ${_f}
-	gcc -o ${.TARGET} ${_f} -lrt
+	${CC} -o ${.TARGET} ${_f} -lrt
 .endfor
 
 .for _f in ${ALL_TESTEXEFILES}