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>

HACKING

@@ -158,6 +158,10 @@ painful. These are:
  * you may assume that right shift of a signed integer duplicates
    the sign bit (ie it is an arithmetic shift, not a logical shift)
 
+In addition, QEMU assumes that the compiler does not use the latitude
+given in C99 and C11 to treat aspects of signed '<<' as undefined, as
+documented in the GNU Compiler Collection manual starting at version 4.0.
+
 7. Error handling and reporting
 
 7.1 Reporting errors to the human user

configure

@@ -1452,7 +1452,7 @@ fi
 gcc_flags="-Wold-style-declaration -Wold-style-definition -Wtype-limits"
 gcc_flags="-Wformat-security -Wformat-y2k -Winit-self -Wignored-qualifiers $gcc_flags"
 gcc_flags="-Wmissing-include-dirs -Wempty-body -Wnested-externs $gcc_flags"
-gcc_flags="-Wendif-labels $gcc_flags"
+gcc_flags="-Wendif-labels -Wno-shift-negative-value $gcc_flags"
 gcc_flags="-Wno-initializer-overrides $gcc_flags"
 gcc_flags="-Wno-string-plus-int $gcc_flags"
 # Note that we do not add -Werror to gcc_flags here, because that would