Implement BCD assist builtins for XL and GCC compatibility.
GCC compat:
```
unsigned int __builtin_cdtbcd (unsigned int);
unsigned int __builtin_cbcdtd (unsigned int);
unsigned int __builtin_addg6s (unsigned int, unsigned int);
```
64BIT XL compat:
```
long long __cdtbcd (long long);
long long __cbcdtd (long long);
long long __addg6s (long long source1, long long source2)
```
