Clean up the build files generated in src/exec

src/exec/Makefile

@@ -4,6 +4,7 @@ EXEC_PROTO := occlum_exec.proto
 SRC_FILES := $(shell find . -type f -name '*.rs') Cargo.toml $(EXEC_PROTO)
 RUST_TARGET_DIR := $(BUILD_DIR)/src/exec/cargo-target
 RUST_OUT_DIR := $(BUILD_DIR)/bin
+EXEC_TARGET := $(RUST_OUT_DIR)/*occlum_exec*
 
 .PHONY: all clean
 
@@ -17,4 +18,5 @@ else
 endif
 
 clean:
-	@cargo clean
+	@cargo clean --target-dir=$(RUST_TARGET_DIR)
+	@-$(RM) -f $(EXEC_TARGET)