Support `make -C compiler`

[oscardssmith]

Now that we have the compiler separated out, it would be useful to have the ability to make -C compiler (like the existing make -C src to make exploration easier when you don't care about recompiling all of Base and the stdlibs just to test your new compiler change.

[Keno]

There's @activate Compiler[:codegen] for that. We could even write that out to a new sysimage, but I don't see why it needs to be in the make system.

[serenity4]

I would guess that the intent with a make command is to have changes be persistent and not require an active REPL to perform, though instead of make we could maybe have a script that wraps around @activate Compiler[:codegen] if we can get it to write out to a new sysimage.

Or, if simple enough, no need to have a script if we can just do e.g. julia -e 'using InteractiveUtils, Compiler; @activate Compiler[:codegen] overwrite_sysimage=true'