64bit: Fix 32bit assumption in astCheckConst()

With the "u" suffix, the literals will be UINTEGERs, that's 64bit on
64bit. So then the lval LONGINT was promoted to 64bit UINTEGER too,
and an unsigned comparison was done, which causes -1 to be treated as
&hFFFFFFFFFFFFFFFF, which is detected as "too big" for a 32bit value...

A signed comparison needs to be done to avoid this, hence the change to
"ll" LONGINT suffix.