[compiler-rt] Add a critical section when flushing gcov counters
Counters can be flushed in a multi-threaded context for example when the process is forked in different threads (https://github.com/llvm/llvm-project/blob/master/llvm/lib/Transforms/Instrumentation/GCOVProfiling.cpp#L632-L663).
In order to avoid pretty bad things, a critical section is needed around the flush.
We had a lot of crashes in this code in Firefox CI when we switched to clang for linux ccov builds and those crashes disappeared with this patch.
Reviewers: marco-c, froydnj, dmajor, davidxl, vsk
Reviewed By: marco-c, dmajor
Subscribers: ahatanak, froydnj, dmajor, dberris, jfb, #sanitizers, llvm-commits, sylvestre.ledru
Tags: #sanitizers, #llvm
Differential Revision: https://reviews.llvm.org/D70910