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/mips_device.go

@@ -121,7 +121,7 @@ func init() {
 
 	pctx.StaticVariable("mipsGccVersion", mipsGccVersion)
 
-	pctx.StaticVariable("mipsGccRoot",
+	pctx.SourcePathVariable("mipsGccRoot",
 		"prebuilts/gcc/${HostPrebuiltTag}/mips/mips64el-linux-android-${mipsGccVersion}")
 
 	pctx.StaticVariable("mipsGccTriple", "mips64el-linux-android")