target/hexagon: suppress unused variable warning

target/hexagon: suppress unused variable warning

This patch manually suppresses a warning for an unused variable
(yynerrs) emitted by bison.

This warning has been triggered for the first time by clang 15.

This patch also disables `-Wextra`, which is not usually adopted in
QEMU. However, clang 15 triggers the warning fixed in this patch even in
absence of `-Wextra`.

Signed-off-by: Alessandro Di Federico <ale@rev.ng>
Signed-off-by: Taylor Simpson <tsimpson@quicinc.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Taylor Simpson <tsimpson@quicinc.com>
Tested-by: Taylor Simpson <tsimpson@quicinc.com>
Message-Id: <20221221155327.1504117-1-ale@rev.ng>