rustc: Change the filename of compressed bitcode

Fixes #12992
Store compressed bitcode files in rlibs with a different extension. Compression doesn't interfere with --emit=bc.
auto merge of #13074 : pczarn/rust/build-rlib, r=alexcrichton

Fixes #12992

I tried to increase the number of deflate's probes. Reduction of 0.5% or 2% is not enough.