Pass PGO_ADDITIONAL_PROFILE_DIRS to soong

This variable can be set in BoardConfig.mk to specify a list of
additional paths that contain PGO profiles.  These directories are
searched before the predefined paths in soong/cc/pgo.go while finding
PGO profiles.

Test: Set this variable in a BoardConfig and verify that such profiles
are found and that these paths are searched before the predefined paths
in soong/cc/pgo.go.

Change-Id: Ied3ebf0ce6e6e2ead57f4828998e9617424e4573
diff --git a/core/soong_config.mk b/core/soong_config.mk
index 639b019..a084f79 100644
--- a/core/soong_config.mk
+++ b/core/soong_config.mk
@@ -133,6 +133,8 @@
 
 $(call add_json_list, NamespacesToExport,                $(PRODUCT_SOONG_NAMESPACES))
 
+$(call add_json_list, PgoAdditionalProfileDirs,          $(PGO_ADDITIONAL_PROFILE_DIRS))
+
 _contents := $(subst $(comma)$(newline)__SV_END,$(newline)}$(newline),$(_contents)__SV_END)
 
 $(file >$(SOONG_VARIABLES).tmp,$(_contents))