Add bccMarkExternalSymbol (to specify external symbol.)

By default libbcc will internalize every symbol, and perform
LTO.  However, in many situations we have to export a specific
function, such as "root" or "init" for RenderScript, so we are
adding bccMarkExternalSymbol.

Change-Id: I136eb8ff68e9ccf8c6595eb470f9718e1cb1e4ab
diff --git a/lib/ExecutionEngine/bcc.cpp b/lib/ExecutionEngine/bcc.cpp
index 6199c16..3d24ac5 100644
--- a/lib/ExecutionEngine/bcc.cpp
+++ b/lib/ExecutionEngine/bcc.cpp
@@ -117,6 +117,12 @@
 }
 
 
+extern "C" void bccMarkExternalSymbol(BCCScriptRef script, char const *name) {
+  BCC_FUNC_LOGGER();
+  unwrap(script)->markExternalSymbol(name);
+}
+
+
 extern "C" int bccPrepareSharedObject(BCCScriptRef script,
                                       char const *cacheDir,
                                       char const *cacheName,