Add coqc -time to Coq 8.4

This cherry-picks the commit that adds -time.  I have tested this, and it compiles and works as expected.

This would be very useful to me while waiting for 8.5 to come out (or on various projects that won't upgrade to 8.4 for a while) to run my bug-finder code more effectively and quickly, as well as to make timing annotations on files.