rts/Profiling: Factor out report generation

Here we move the actual report generation logic to
`rts/ProfilerReport.c`. This break is actually quite clean,

    void writeCCSReport( FILE *prof_file, CostCentreStack const *ccs,
                         ProfilerTotals totals );

This is more profiler refactoring in preparation for machine-readable
output.

Test Plan: Validate

Reviewers: austin, erikd, simonmar

Reviewed By: simonmar

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D3097