Update core/Makefile to add variable PRODUCT_EXTRA_STUB_LIBRARIES.

This allows products to specify additional device-specific stub libraries that should be included in the system_linker_config. This is useful for products that need to include additional stub libraries that are not picked up by Soong automatically.

Bug: 361441210
Change-Id: Ia18c476e249ac5b14b43819fa0a4f0059dc620de
diff --git a/core/product.mk b/core/product.mk
index 4c23e5d..b07e6e0 100644
--- a/core/product.mk
+++ b/core/product.mk
@@ -499,6 +499,10 @@
 # If set, build would generate system image from Soong-defined module.
 _product_single_value_vars += PRODUCT_SOONG_DEFINED_SYSTEM_IMAGE
 
+# List of stub libraries specific to the product that are already present in the system image and
+# should be included in the system_linker_config.
+_product_list_vars += PRODUCT_EXTRA_STUB_LIBRARIES
+
 .KATI_READONLY := _product_single_value_vars _product_list_vars
 _product_var_list :=$= $(_product_single_value_vars) $(_product_list_vars)