Upgrade liblibc to latest upstream version.

This version of liblibc is a prerequisite for getting libstd to build on
CloudABI.

diff --git a/src/liblibc b/src/liblibc
index ef9eefb6df3f3a2cb989e8050519661faa7d7118..2b4cd1016bdba92becb4f982a4dcb18fe6653bc4 160000 (submodule)
--- a/src/liblibc
+++ b/src/liblibc
@@ -1 +1 @@
-Subproject commit ef9eefb6df3f3a2cb989e8050519661faa7d7118
+Subproject commit 2b4cd1016bdba92becb4f982a4dcb18fe6653bc4