hadrian: Fix library-dirs, dynamic-library-dirs and static-library-dirs in inplace .conf files

Previously we were just throwing away the contents of the library-dirs
fields but really we have to do the same thing as for include-dirs,
relativise the paths into the current working directory and maintain any
extra libraries the user has specified.

Now the relevant section of the rts.conf file looks like:

```
library-dirs:
    ${pkgroot}/../rts/build
    ${pkgroot}/../../..//_build/stage1/rts/build
    /nix/store/av4c0fib4rkmb6sa1074z0rb1ciria5b-gperftools-2.10/lib
    /nix/store/2infxahfp9lj084xn3q9ib5ajks8447i-libffi-3.4.4/lib

library-dirs-static:
    ${pkgroot}/../rts/build
    ${pkgroot}/../../..//_build/stage1/rts/build
    /nix/store/av4c0fib4rkmb6sa1074z0rb1ciria5b-gperftools-2.10/lib
    /nix/store/2infxahfp9lj084xn3q9ib5ajks8447i-libffi-3.4.4/lib

dynamic-library-dirs:
    ${pkgroot}/../rts/build
    ${pkgroot}/../../..//_build/stage1/rts/build
    /nix/store/av4c0fib4rkmb6sa1074z0rb1ciria5b-gperftools-2.10/lib
    /nix/store/2infxahfp9lj084xn3q9ib5ajks8447i-libffi-3.4.4/lib
```

Fixes #22209