Auto merge of #26018 - nrc:codegen, r=@nikomatsakis

Closes #19826

r? @nikomatsakis 

There is still some work to do until parallel codegen is perfect - we don't pass make check due to missing the symbol for registering a macro. But we do bootstrap with codegen_units=4 and pass make check without codegen_units, so I think it should land.

For the curious, make rustc-stage2 -j8:

```
codegen_units=1:

real  9m18.074s
user  11m59.858s
sys 0m13.281s


codegen_units=4:

real  6m3.672s
user  13m5.474s
sys 0m12.146s
```

Which is a 33% speedup :-)

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