compiler: Always load GHC.Data.FastString optimised into GHCi

The FastString table is shared between the boot compiler and interpreted compiler. Therefore it's very important the representation of FastString matches in both cases. Otherwise, the interpreter will read a FastString from the shared variable but place the fields in the wrong place which leads to segfaults.

Ideally this state would not be shared, but for now we can always compile both with -O2 and this leads to a working interpreter.