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.