toolchain: kvm: add needed empty lib/ folder

This (empty) folder is needed for the linker to find lib64/ because
its path contains something like
"x86_64-buildroot-linux-gnu/lib/../lib64".

I added a Readme in the folder because git doesn't like empty folders.

Change-Id: I6c81a4c90792925bbc3c63d3a7fe2b99a54c4601
diff --git a/x86_64-buildroot-linux-gnu/lib/Readme b/x86_64-buildroot-linux-gnu/lib/Readme
new file mode 100644
index 0000000..38bacd6
--- /dev/null
+++ b/x86_64-buildroot-linux-gnu/lib/Readme
@@ -0,0 +1,4 @@
+This (empty) folder is needed for the linker to find lib64/ because its path
+contains something like "x86_64-buildroot-linux-gnu/lib/../lib64".
+
+This Readme is needed because git doesn't like empty folder.