FR: add a system allowing to define the linker to invoke

Because of #31066 (closed) if the system is configured to use ld.gold as linker, FPC fails to link programs.

For example here, on Fedora linux, I have to use alternatives --config ld each time I start Lazarus.

There should be a way to configure the particular linker that FPC invokes, either (sorted by descending personnal preference)

• an entry in the configuration file
• using the LD env variable
• a compiler flag, e.g fpc <usual args...> --linker=ld.bfd