Disable warn about left shifts of negative values

It seems like there's no good reason for the compiler to exploit the
undefinedness of left shifts.  GCC explicitly documents that they do not
use at all this possibility and, while they also say this is subject
to change, they have been saying this for 10 years (since the wording
appeared in the GCC 4.0 manual).

Disable these warnings by passing in -Wno-shift-negative-value.

Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: Markus Armbruster <armbru@redhat.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
[pranith: forward-port part of patch to 2.7]
Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
