libvixl: Avoid std::abs() of 64-bit type

libvixl: Avoid std::abs() of 64-bit type

The std::abs() function did not get a version that works on
'long long' until C++11. Avoid it, so that we can compile on
32-bit platforms (where int64_t is 'long long') with older
compilers (which don't support C++11).

Reported-by: Franz-Josef Haider <Franz-Josef.Haider@student.uibk.ac.at>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1453739429-31477-1-git-send-email-peter.maydell@linaro.org