gcov: use __gcov_dump + __gcov_reset instead of __gcov_flush

__gcov_flush was removed in gcc11.

Since gcc11 __gcov_dump calls __gcov_lock at the start and
__gcov_unlock before returning. Same is true for __gcov_reset.
Because of that using __gcov_reset right after __gcov_dump since gcc11
is the same as using __gcov_flush before gcc11.

Closes #7302

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