Don't build empty compiler-rt.lib on Windows MSVC at all

[angelsl]

Depends on #28611, #28633, and blocked on next snapshot containing those two.

Once our bootstrap snapshot doesn't link to compiler-rt on MSVC, we can stop building compiler-rt.lib completely.

r? @alexcrichton

[rust-highfive]

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[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.