This is still not properly resolving <https://gcc.gnu.org/PR85463>
'[nvptx] "exit" in offloaded region doesn't terminate process', but is
one step into that direction, and allows for simplifying some GCC code.
... so that all of 'exit', '_exit', '_Exit' work. 'exit' thus becomes the
standard 'newlib/libc/stdlib/exit.c' -- and functions registered via 'atexit'
are now called at return from 'main' or manual 'exit' invocation.