HPCC-12644 Fix problems in 32bit folding int64 return types

intresult was signed, causing the '+' to decrement the value, resulting
in a value 2^32 smaller than it should have been.

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Those functions aren't in the eclrtl already.  I could add them, but I doubt they would reveal anything.