Switch [stout] to using compiler intrinsics for unreachable, exit, and abort

Use compiler intrinsics for unreachable, exit, and abort
Makes the functions not need to pretend to return something while
still silencing the compiler warnings.

Replaces 25191 (I originally authored the patch, but Patrick was managing it for various reasons). Incorporates the fix suggested by Ben Mahler. Fixes Dominic's build issue.

Review: https://reviews.apache.org/r/26100
Switch [libprocess] to using compiler intrinsics for unreachable, exit, and abort

Use compiler intrinsics for unreachable, exit, and abort
Makes the functions not need to pretend to return something while
still silencing the compiler warnings.

Replaces 25192

Review: https://reviews.apache.org/r/26101
Switch [mesos] to using compiler intrinsics for unreachable, exit, and abort

Use compiler intrinsics for unreachable, exit, and abort
Makes the functions not need to pretend to return something while
still silencing the compiler warnings.

Replaces 25193

Review: https://reviews.apache.org/r/26103