Fix TestExec after r334783

The second makefile that was added has implicit rules which meant
that secondprog.cpp would be built once into a secondprog binary,
but it would also be compiled as a.out overwriting the main binary.
This lead to spurious failures.

This commit simplifies the Makefile to build only once with the correct
executable name.

llvm-svn: 334861

lldb/packages/Python/lldbsuite/test/functionalities/exec/secondprog.mk

 LEVEL = ../../make
 
 CXX_SOURCES := secondprog.cpp
-
-all: secondprog
-
-secondprog:
-	$(CXX) $(CXXFLAGS) -o secondprog $(SRCDIR)/secondprog.cpp
-
-clean::
-	rm -rf secondprog secondprog.dSYM
+EXE = secondprog
 
 include $(LEVEL)/Makefile.rules