Add multi-threading support for LLVM profile merging

[asthetik]

This PR fixes issue #1116 by passing the --threads parameter from grcov to the underlying llvm-profdata merge command

[marco-c]

A couple of things:

1. we should do the same for llvm-cov export;
2. since llvm-cov and llvm-profdata actually do this by default when --num-threads is 0 (and they autodetect the right number of threads to use), if the grcov parameter is not passed explicitly by the user, let's rely on their autodetection instead of ours.