linux-user/i386: Split out gen_signal

linux-user/i386: Split out gen_signal

This is a bit tidier than open-coding the 5 lines necessary
to initialize the target_siginfo_t.  In addition, this zeros
the remaining bytes of the target_siginfo_t, rather than
passing in garbage.

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20200213032223.14643-3-richard.henderson@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>