testsuite: Fix up gfortran.dg/gomp/strictly*.f90 testcases

testsuite: Fix up gfortran.dg/gomp/strictly*.f90 testcases

While these testcases are dg-do compile only, I think it is better not to
give users bad examples and avoid unnecessary data races in testcases (unless
it is exactly what we want to test).  Perhaps one day we'll do some analysis
and warn about data races...

2021-10-21  Jakub Jelinek  <jakub@redhat.com>

	* gfortran.dg/gomp/strictly-structured-block-1.f90: Use call do_work
	instead of x = x + 1 in places where the latter could be a data race.
	* gfortran.dg/gomp/strictly-structured-block-2.f90: Likewise.
	* gfortran.dg/gomp/strictly-structured-block-3.f90: Likewise.