[Pal] Use := to get correct dir in Makefile.Test

Pal/src/Makefile.Test

@@ -1,4 +1,4 @@
-DIR = $(dir $(lastword $(MAKEFILE_LIST)))
+DIR := $(dir $(lastword $(MAKEFILE_LIST)))
 expand_target = $(1)
 
 ifeq ($(PAL_HOST),)

Pal/src/host/Linux-SGX/Makefile.Test

@@ -1,4 +1,4 @@
-SGX_DIR = $(abspath $(dir $(lastword $(MAKEFILE_LIST))))
+SGX_DIR := $(abspath $(dir $(lastword $(MAKEFILE_LIST))))
 RUNTIME_DIR = $(SGX_DIR)/../../../../Runtime
 
 LIBPAL = $(RUNTIME_DIR)/libpal-Linux-SGX.so