For compilers (such as Microsoft VC++) which don't allow "LL" after a
constant to make it 64-bit, this patch declares all such constants as
BX_CONST64(value). Then in config.in, a switch called
BX_64BIT_CONSTANTS_USE_LL controls whether the macro puts the
LL's in or not. Configure sets the macro, if you're on a platform
that can run such things.