More performance improvements

Instead of using `x := $(x) foo`, make sure `x` is initialized first,
then use `x += foo`.

For a aosp-master/aosp_crosshatch-userdebug build, this takes the build
from 49s to 33s (no significant difference in peak memory)

Bug: 158488548
Test: build-aosp_crosshatch.ninja is the same before/after
Change-Id: I41fb7611030dd3ba824f896a9c9eadbdd218f464
diff --git a/core/soong_cc_prebuilt.mk b/core/soong_cc_prebuilt.mk
index 6317b53..986609b 100644
--- a/core/soong_cc_prebuilt.mk
+++ b/core/soong_cc_prebuilt.mk
@@ -51,7 +51,7 @@
 
 ifneq ($(filter STATIC_LIBRARIES SHARED_LIBRARIES HEADER_LIBRARIES,$(LOCAL_MODULE_CLASS)),)
   # Soong module is a static or shared library
-  EXPORTS_LIST := $(EXPORTS_LIST) $(intermediates)
+  EXPORTS_LIST += $(intermediates)
   EXPORTS.$(intermediates).FLAGS := $(LOCAL_EXPORT_CFLAGS)
   EXPORTS.$(intermediates).DEPS := $(LOCAL_EXPORT_C_INCLUDE_DEPS)
 
@@ -61,7 +61,7 @@
     $(my_all_targets): $(LOCAL_BUILT_MODULE).toc
   endif
 
-  SOONG_ALREADY_CONV := $(SOONG_ALREADY_CONV) $(LOCAL_MODULE)
+  SOONG_ALREADY_CONV += $(LOCAL_MODULE)
 
   my_link_type := $(LOCAL_SOONG_LINK_TYPE)
   my_warn_types :=