target-i386: Use 1UL for bit shift

Fix undefined behavior detected by clang runtime check:

  qemu/target-i386/cpu.c:1494:15: runtime error:
    left shift of 1 by 31 places cannot be represented in type 'int'

While doing that, add extra parenthesis for clarity.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
