[DO NOT MERGE] Enable parallel compiler by default#75651

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Aaron1011 wants to merge 2 commits intorust-lang:masterfrom

Aaron1011:parallel-perf

Commits on Aug 17, 2020

[DO NOT MERGE] Enable parallel compiler by default
Aaron1011
committed

Commits on Aug 29, 2020

default to 12 threads
Mark-Simulacrum
committed