[sancov] Handle fork.

Reset coverage data on fork().
For memory-mapped mode (coverage_direct=1) this helps avoid loss of data
(before this change two processes would write to the same file simultaneously).
For normal mode, this reduces coverage dump size, because PCs from the parent
process are no longer inherited by the child.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@210180 91177308-0d34-0410-b5e6-96231b3b80d8