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

cc/arm_device.go

@@ -154,7 +154,7 @@ func init() {
 
 	pctx.StaticVariable("armGccVersion", armGccVersion)
 
-	pctx.StaticVariable("armGccRoot",
+	pctx.SourcePathVariable("armGccRoot",
 		"prebuilts/gcc/${HostPrebuiltTag}/arm/arm-linux-androideabi-${armGccVersion}")
 
 	pctx.StaticVariable("armGccTriple", "arm-linux-androideabi")