Add dummy undefined symbols to indicate ways

Currently, "way" information is not recorded into object files; it is only recorded into 'hi' files. This means if you are using, say, rts/Linker.c (say, if you're GHCi or a dynamic plugins package), if you mess up and load an object file compiled the wrong way, you will be very sad.

I think a cute trick we could play is to define a bunch of dummy symbols per-way, and add them as undefined symbols to objects, to indicate what way they were compiled, e.g. with the -u linker flag. Various RTS's export the symbols of ways they support, and attempting to link an object with an incompatible RTS results in a link error.

Are there any problems with this scheme? If not, I'll go ahead and implement it.

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Priority	normal
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