testsuite: Add profile_update_atomic check to gcov-20.c [PR114614]

testsuite: Add profile_update_atomic check to gcov-20.c [PR114614]

As PR114614 shows, the newly added test case gcov-20.c by
commit r14-9789-g08a52331803f66 failed on targets which do
not support atomic profile update, there would be a message
like:

  warning: target does not support atomic profile update,
           single mode is selected

Since the test case adopts -fprofile-update=atomic, it
requires effective target check profile_update_atomic, this
patch is to add the check accordingly.

	PR testsuite/114614

gcc/testsuite/ChangeLog:

	* gcc.misc-tests/gcov-20.c: Add effective target check
	profile_update_atomic.