Build fake compiler-rt on MSVC

[angelsl]

Depends on #28611.

Since we no longer need compiler-rt, don't build it. This removes the dependency on MinGW-W64 completely.

We build an empty compiler-rt.lib because our bootstrap snapshot still links to compiler-rt.lib when compiling an output.

[rust-highfive]

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[angelsl]

r? @alexcrichton

[alexcrichton]

Thanks for the PR! I'm not sure, however, that we'll be able to land #28611 soon, so I'm gonna close this waiting for that to be resolved. If that's merged soon, however, I'll try to reopen this.