[patch] MCSymbols for FastISel

[Rafael Espíndola]

This is basically a leftover from my previous patch.

The summary is that it moves the mangling earlier and replaces a few
calls to .addExternalSymbol with addSym.

I originally wanted to replace all the uses of addExternalSymbol with
addSym, but noticed it was not needed for the previous patch and that
it was a lot of work.

I still think it would be a good thing. This patch is just the part I
already had implemented.

Cheers,
Rafael
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