Webkit FTBFS on s390x and powerpc because it tries to use the macroassembler when it should not

As discussed in [0] Qt Webkit is FTBFS in s390x [1] and powerpc [2] because it's trying to compile with MacroAssembler support while it shouldn't.

Of course, feel free to tell me where should I look to try to get a fix for this, as I recognize not everyone has access to this arches.

[0] <http://lists.qt-project.org/pipermail/development/2014-February/015616.html>
[1] <<https://buildd.debian.org/status/fetch.php?pkg=qtwebkit-opensource-src&arch=s390x&ver=5.2.0%2Bdfsg1-1&stamp=1391444133>>
[2] <https://buildd.debian.org/status/fetch.php?pkg=qtwebkit-opensource-src&arch=powerpc&ver=5.2.0%2Bdfsg-2&stamp=1388005710>