Revert "[lsan] On Fuchsia, don't use atexit hook for leak checks"

This reverts commit 0caad9fe.
This reverts commit c96d0cce.

Causes linker errors which were not fixed by the subsequent commit
either:

/home/nikic/llvm-project/compiler-rt/lib/asan/asan_rtl.cpp:503: error: undefined reference to '__asan::InstallAtExitCheckLeaks()'