segfault when building for aarch64 target

[Ard Biesheuvel]

When Clang itself is built with GCC 5 or later, it may crash when building for the aarch64 architecture if the following patch is not applied

https://www.marc.info/?l=llvm-commits&m=142918476428427&w=4

This applies to wily only: vivid uses GCC 4.9, and xenial already has the above patch applied.

It is currently unknown whether upstream clang will do another release on the 3.6 branch, but if that happens, this fix will be included as well.

Reproduce:

clang -target aarch64 -c -o DxePcdLib.o DxePcdLib.i

http://people.linaro.org/~ard.biesheuvel/DxePcdLib.i