Generate NDK sysroots from the platform build.

The list of migrated libraries is currently empty. Libraries will be
migrated as follow up patches.

Test: Migrated libc to this system and everything still builds.
      build.ninja shows libraries being built and used and headers are
      collected for the sysroot.
Bug: http://b/27533932
Change-Id: Iaba00543c1390f432befe0eed768ed3fbb8a9b96
diff --git a/cc/makevars.go b/cc/makevars.go
index 0e75329..ae95a18 100644
--- a/cc/makevars.go
+++ b/cc/makevars.go
@@ -50,6 +50,8 @@
 	ctx.StrictRaw("SRC_HEADERS", strings.Join(includes, " "))
 	ctx.StrictRaw("SRC_SYSTEM_HEADERS", strings.Join(systemIncludes, " "))
 
+	ctx.Strict("NDK_MIGRATED_LIBS", strings.Join(ndkMigratedLibs, " "))
+
 	hostTargets := ctx.Config().Targets[android.Host]
 	makeVarsToolchain(ctx, "", hostTargets[0])
 	if len(hostTargets) > 1 {
@@ -198,6 +200,7 @@
 		ctx.Strict(makePrefix+"STRIP", gccCmd(toolchain, "strip"))
 		ctx.Strict(makePrefix+"GCC_VERSION", toolchain.GccVersion())
 		ctx.Strict(makePrefix+"NDK_GCC_VERSION", toolchain.GccVersion())
+		ctx.Strict(makePrefix+"NDK_TRIPLE", toolchain.ClangTriple())
 	}
 
 	ctx.Strict(makePrefix+"TOOLCHAIN_ROOT", toolchain.GccRoot())