[gcov] Add __gcov_dump/__gcov_reset and delete __gcov_flush

GCC r187297 (2012-05) introduced `__gcov_dump` and `__gcov_reset`.
  `__gcov_flush = __gcov_dump + __gcov_reset`

The resolution to https://gcc.gnu.org/PR93623 ("No need to dump gcdas when forking" target GCC 11.0) removed the unuseful and undocumented __gcov_flush.

Close PR38064.

Reviewed By: calixte, serge-sans-paille

Differential Revision: https://reviews.llvm.org/D83149