Replaced `abort()` with `_exit()` in `ChildHook::SUPERVISOR`.
Replaced `abort()` with `_exit()` in `ChildHook::SUPERVISOR`.

Previously, `abort()` was called in `SUPERVISOR` hook when child
process exited with an error code, or `waitpid()` failed, or parent
process exited. All these cases shouldn't lead to abnormal program
termination with coredumps.

Review: https://reviews.apache.org/r/60598/

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