disas/libvixl: Suppress gcc 4.6.3 sign-compare warnings

disas/libvixl: Suppress gcc 4.6.3 sign-compare warnings

The VIXL code includes some equality comparisons between signed
and unsigned types. Modern gcc and clang do not complain about
these, but older versions of gcc such as gcc 4.6.3 do. Since
libvixl is an upstream library, the simplest approach is to
suppress the warnings by applying -Wno-sign-compare to the
relevant files.

(GCC 4.6 is not quite yet irrelevant for us; it is the gcc
shipped with Ubuntu Precise, for example, which is an LTS
release not yet out of its support period.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1452604204-27202-1-git-send-email-peter.maydell@linaro.org
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>