Update Clang for rebase to r212749.

This also fixes a small issue with arm_neon.h not being generated always.

Includes a cherry-pick of:
r213450 - fixes mac-specific header issue
r213126 - removes a default -Bsymbolic on Android

Change-Id: I2a790a0f5d3b2aab11de596fc3a74e7cbc99081d
diff --git a/www/get_involved.html b/www/get_involved.html
index 5d16db9..06fee59 100644
--- a/www/get_involved.html
+++ b/www/get_involved.html
@@ -21,6 +21,11 @@
 the development of the project to see it progress.
 </p>
 
+<h2>Contribute</h2>
+
+See the <a href="hacking.html">hacking</a> document for information on how
+to author patches.
+
 <h2>Follow what's going on</h2>
 
 <p>Clang is a subproject of the <a href="http://llvm.org">LLVM Project</a>, but