Exposing __gcov_flush increases parity with libgcov project, and exposes
the symbol for use with applications that need to flush gcov information,
counters, etc, when exiting via calls which don't invoke __gcov_flush
indirectly, like _exit(2).
Update the tests to use the new header, gcov.h, and fix the tests so
they compile cleanly on [more] platforms by including the appropriate