Check out the right versions of libunwind/libcxxabi/libcxx before building at all

This makes sure that libunwind and libcxxabi actually get built
against the same headers that are checked out later.

build-libcxx.sh

@@ -24,15 +24,29 @@ if [ ! -d libcxx ]; then
     git clone -b master https://github.com/llvm-mirror/libcxx.git
     CHECKOUT_LIBCXX=1
 fi
+if [ -n "$SYNC" ] || [ -n "$CHECKOUT_LIBUNWIND" ]; then
+    cd libunwind
+    [ -z "$SYNC" ] || git fetch
+    git checkout 2ddcf2461daa5d61c543474aed06b12a8b9ad816
+    cd ..
+fi
+if [ -n "$SYNC" ] || [ -n "$CHECKOUT_LIBCXXABI" ]; then
+    cd libcxxabi
+    [ -z "$SYNC" ] || git fetch
+    git checkout 05ba3281482304ae8de31123a594972a495da06d
+    cd ..
+fi
+if [ -n "$SYNC" ] || [ -n "$CHECKOUT_LIBCXX" ]; then
+    cd libcxx
+    [ -z "$SYNC" ] || git fetch
+    git checkout 16eb426933cdb1c54fd6fbba5fc06cc5a4ca52b3
+    cd ..
+fi
 
 LIBCXX=$(pwd)/libcxx
 MERGE_ARCHIVES=$(pwd)/merge-archives.sh
 
 cd libunwind
-if [ -n "$SYNC" ] || [ -n "$CHECKOUT_LIBUNWIND" ]; then
-    [ -z "$SYNC" ] || git fetch
-    git checkout 2ddcf2461daa5d61c543474aed06b12a8b9ad816
-fi
 for arch in $ARCHS; do
     mkdir -p build-$arch
     cd build-$arch
@@ -67,10 +81,6 @@ done
 cd ..
 
 cd libcxxabi
-if [ -n "$SYNC" ] || [ -n "$CHECKOUT_LIBCXXABI" ]; then
-    [ -z "$SYNC" ] || git fetch
-    git checkout 05ba3281482304ae8de31123a594972a495da06d
-fi
 for arch in $ARCHS; do
     mkdir -p build-$arch
     cd build-$arch
@@ -101,10 +111,6 @@ done
 cd ..
 
 cd libcxx
-if [ -n "$SYNC" ] || [ -n "$CHECKOUT_LIBCXX" ]; then
-    [ -z "$SYNC" ] || git fetch
-    git checkout 16eb426933cdb1c54fd6fbba5fc06cc5a4ca52b3
-fi
 for arch in $ARCHS; do
     mkdir -p build-$arch
     cd build-$arch