Bug#764195: gcc-4.9: problems with precompiled headers on arm64

[Edmund Grimley Evans <edmund.grimley.evans@gmail.com>]

Package: gcc-4.9
Version: 4.9.1-16

There are a couple of Debian packages that failed to build on arm64
apparently because of problems with precompiled headers:

qtbase-opensource-src
See http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=762702

aegisub
See http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=764187

It's difficult to extract a small test case, and it's presumably
impossible to reproduce the problem with preprocessed source as GCC's
standard internal compiler error message suggests.