grcov --threads parameter is not passed to sub commands

[vanhauser-thc]

Problem:
grcov executes llvm-profdata and llvm-report if profdata is present, but does not pass on the number of threads to be used. Especially for profdata this slows down llvm-profdata a lot (I have a 50% speed increase from 60 minutes to 90 minutes because of that).

[marco-c]

Thanks for reporting, I'd gladly accept a PR for fixing this (https://github.com/mozilla/grcov/blob/master/src/llvm_tools.rs is the file to touch to fix it).