sim: cgen: rework DI macros to avoid signed left shifts

sim: cgen: rework DI macros to avoid signed left shifts

The cgen code uses DI as int64_t and UDI as uint64_t.  The DI macros
are used to construct 64-bit values from 32-bit values (for the low
and high parts).  The MAKEDI macro casts the high 32-bit value to a
signed 32-bit value before shifting.  If this created a negative
value, this would be undefined behavior according to the C standard.
All we care about is shifting the 32-bits as they are to the high
32-bits, not caring about sign extension (since there's nothing left
to shift into), and the low 32-bits being empty.  This is what we
get from shifting an unsigned value, so cast it to unsigned 32-bit
to avoid undefined behavior.

While we're here, change the SETLODI macro to truncate the lower
value to 32-bits before we set it.  If it was passing in a 64-bit
value, those high bits would get included too, and that's not what
we want.

Similarly, tweak the SETHIDI macro to cast the value to an unsigned
64-bit instead of a signed 64-bit.  If the value was only 32-bits,
the behavior would be the same.  If it happened to be signed 64-bit,
it would trigger the undefined behavior too.