Use `Path` instead of string for file paths

This centralizes verification and common operations, like converting the
path to a source file to the path for a built object.

It also embeds the configuration knowledge into the path, so that we can
remove "${SrcDir}/path" from the ninja file. When SrcDir is '.', that
leads to paths like './path' instead of just 'path' like make is doing,
causing differences in compiled binaries.

Change-Id: Ib4e8910a6e867ce1b7b420d927c04f1142a7589e
diff --git a/cc/x86_device.go b/cc/x86_device.go
index 6dfbd6a..a44b293 100644
--- a/cc/x86_device.go
+++ b/cc/x86_device.go
@@ -139,7 +139,7 @@
 
 	pctx.StaticVariable("x86GccVersion", x86GccVersion)
 
-	pctx.StaticVariable("x86GccRoot",
+	pctx.SourcePathVariable("x86GccRoot",
 		"prebuilts/gcc/${HostPrebuiltTag}/x86/x86_64-linux-android-${x86GccVersion}")
 
 	pctx.StaticVariable("x86GccTriple", "x86_64-linux-android")