target-arm/arm-semi.c: In SYS_HEAPINFO use correct type for 'limit'

In commit f5666418c4 most of the SYS_HEAPINFO implementation was
fixed to use target_ulong rather than uint32_t, but the 'limit'
variable was not changed.

Reported-by: Laurent Desnogues <laurent.desnogues@gmail.com>
Reviewed-by: Laurent Desnogues <laurent.desnogues@gmail.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1467650942-28706-1-git-send-email-peter.maydell@linaro.org
