__cxa_finalize: skip fflush call on dlclose

In __cxa_finalize, only call fflush(NULL) when the program is exiting, not
when a library is unloaded with dlclose. This change restores behavior
from 2015.

Flushing output is needed when the program exits, but flushing everything
is hazardous at other times because it can block -- fflush(NULL) locks
every file, so it also blocks on read operations.

Bug: http://b/130655235
Test: manual
Change-Id: I2f5ecffa6724bfd98a93d145ab5313c793c01ae6
(cherry picked from commit c5d8c6c6e47a7f5fd0c3db2a48004be1030cf753)