linux-user: fix gcc-4.9 compiler error on __{get,put]}_user

linux-user: fix gcc-4.9 compiler error on __{get,put]}_user

gcc-4.9 finds unused operand:

linux-user/syscall.c: In function ‘host_to_target_stat64’:
linux-user/qemu.h:301:19: error: right-hand operand of comma expression
has no effect [-Werror=unused-value]
      ((hptr), (x)), 0)

Just removing the rh operand is no good, it will error in later:

linux-user/main.c: In function ‘arm_kernel_cmpxchg64_helper’:
linux-user/qemu.h:330:15: error: void value not ignored as it ought to be
         __ret = __put_user((x), __hptr);    \

Thus, remove setting __ret from __get_user and __put_user, as and
set the right hand operand to (void)0 to make it clear that these
return never nothing.

This commit depends on the signal.c cleanup, to ensure bisectable
version history.

Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Cc: Richard Henderson <rth@twiddle.net>