T10518: Ensure literal has 64-bit type

Otherwise we get a C-- lint error due to mismatched RHS and
variable types.

testsuite/tests/codeGen/should_compile/T10518.cmm

 foo() {
     bits64 a;
-    a = 0x10000000000000000; // overflows 64 bits
+    a = (0x10000000000000000 :: bits64); // overflows 64 bits
     return (a);
 }