linux-user: Fix clang warning for nios2-linux-user code

The clang in Ubuntu 18.04 (10.0.0-4ubuntu1) produces a warning
on the code added in commit f5ef0e51 where we use a
shifted expression in a boolean context:

../../linux-user/elfload.c:2423:16: error: converting the result of '<<' to a boolean always evaluates to true [-Werror,-Wtautological-constant-compare]
    } else if (LO_COMMPAGE) {
               ^
../../linux-user/elfload.c:1102:22: note: expanded from macro 'LO_COMMPAGE'
#define LO_COMMPAGE  TARGET_PAGE_SIZE
                     ^
/mnt/nvmedisk/linaro/qemu-from-laptop/qemu/include/exec/cpu-all.h:231:31: note: expanded from macro 'TARGET_PAGE_SIZE'
#define TARGET_PAGE_SIZE   (1 << TARGET_PAGE_BITS)
                              ^
1 error generated.

The warning is bogus because whether LO_COMMPAGE is zero or not
depends on compile-time ifdefs; shut the compiler up by adding
an explicit comparison to zero.

Fixes: f5ef0e51 ("linux-user/nios2: Map a real kuser page")
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-id: 20220111082900.3341274-1-peter.maydell@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>