Add Apache 2.0 exception to enable GPLv2 compatibility

These exceptions are used by LLVM to enable Apache 2.0 license code to
be used with GPLv2 licensed code.

See some discussion here
https://lists.llvm.org/pipermail/llvm-dev/2016-September/104778.html for
background.

These exceptions have been taken from the LLVM license:
https://llvm.org/LICENSE.txt

LICENSE

@@ -173,4 +173,19 @@
       incurred by, or claims asserted against, such Contributor by reason
       of your accepting any such warranty or additional liability.
 
+---- Exceptions to the Apache 2.0 License ----
+
+As an exception, if, as a result of your compiling your source code, portions of
+this Software are embedded into an Object form of such source code, you may
+redistribute such embedded portions in such Object form without complying with
+the conditions of Sections 4(a), 4(b) and 4(d) of the License.
+
+In addition, if you combine or link compiled forms of this Software with
+software that is licensed under the GPLv2 ("Combined Software") and if a court
+of competent jurisdiction determines that the patent provision (Section 3), the
+indemnity provision (Section 9) or other Section of the License conflicts with
+the conditions of the GPLv2, you may retroactively and prospectively choose to
+deem waived or otherwise exclude such Section(s) of the License, but only in
+their entirety and only with respect to the Combined Software.
+
    END OF TERMS AND CONDITIONS