Fix OpenACC gang-redundant execution in 'libgomp.oacc-fortran/privatized-ref-2.f90'

This was a latent problem, and this commit here now resolves a regression that
after recent commit a78b1ab1
"amdgcn: Tune default OpenMP/OpenACC GPU utilization" we had (only) seen on a
GCN offloading '-march=gfx908' system:

    {+WARNING: program timed out.+}
    [-PASS:-]{+FAIL:+} libgomp.oacc-fortran/privatized-ref-2.f90 -DACC_DEVICE_TYPE_radeon=1 -DACC_MEM_SHARED=0 -foffload=amdgcn-amdhsa  -O0  execution test

Same for other optimization levels.

Make sure that we're not executing non-parallelized code in gang-redundant
mode, by putting these parts into their own 'parallel' constructs, which then
default to 'num_gangs(1)'.

	libgomp/
	* testsuite/libgomp.oacc-fortran/privatized-ref-2.f90: Fix OpenACC
	gang-redundant execution.