Replace android.BuildOs with Config.BuildOS

Replace the android.BuildOs constant with Config.BuildOS so that it
can vary based on the product config.

Bug: 190084016
Test: all Soong tests
Change-Id: Ia67f872d8b2ab788747a22e3a9659dc21c9775cd
diff --git a/cc/config/global.go b/cc/config/global.go
index dfbe6c4..55e0d79 100644
--- a/cc/config/global.go
+++ b/cc/config/global.go
@@ -15,6 +15,7 @@
 package config
 
 import (
+	"runtime"
 	"strings"
 
 	"android/soong/android"
@@ -282,7 +283,7 @@
 var pctx = android.NewPackageContext("android/soong/cc/config")
 
 func init() {
-	if android.BuildOs == android.Linux {
+	if runtime.GOOS == "linux" {
 		commonGlobalCflags = append(commonGlobalCflags, "-fdebug-prefix-map=/proc/self/cwd=")
 	}