Use architecture-dependent UBaseType_t instead of fixed type for
ullMachineTimerCompareRegisterBase. This type is defined to uint32_t or
uint64_t based on XLEN, resolving warnings on 32-bit platforms.
Reported by landretk@ on the PR FreeRTOS/FreeRTOS-Kernel#1176.
Signed-off-by: Gaurav Aggarwal <aggarg@amazon.com>