Fix detection of thumb STL libraries.

I think the CWD must have changed when I moved us to using
build_support.py, which means we no longer find the thumb libraries
and don't try to package them.

Note that we don't need to correct the paths that we actually add for
packaging because pack_archive will cd into the right directory for
that operation anyway.

Change-Id: Ia8ee9bc056d8570829f77b0c19d050a71cb564ea
diff --git a/build/tools/build-cxx-stl.sh b/build/tools/build-cxx-stl.sh
index edcc0eb..62f7399 100755
--- a/build/tools/build-cxx-stl.sh
+++ b/build/tools/build-cxx-stl.sh
@@ -671,7 +671,7 @@
         FILES=""
         LIB_SUFFIX="$(get_lib_suffix_for_abi $ABI)"
         for LIB in ${CXX_STL_LIB}_static.a ${CXX_STL_LIB}_shared${LIB_SUFFIX}; do
-	    if [ -d "$CXX_STL_SUBDIR/libs/$ABI/thumb" ]; then
+            if [ -d "$ANDROID_NDK_ROOT/$CXX_STL_SUBDIR/libs/$ABI/thumb" ]; then
                 FILES="$FILES $CXX_STL_SUBDIR/libs/$ABI/thumb/$LIB"
             fi
             FILES="$FILES $CXX_STL_SUBDIR/libs/$ABI/$LIB"