[libgomp, openacc, testsuite] Fix async logic in lib-12.f90

[libgomp, openacc, testsuite] Fix async logic in lib-12.f90

In testcase lib-12.f90, all acc_async_test calls are placed in a location
where they are not guaranteed to succeed, which explains why there's an xfail
for the lower optimization levels.

This patch fixes the problem by moving the acc_async_test calls to the correct
locations.

Reg-tested on x86_64 with nvptx accelerator.

2018-07-26  Tom de Vries  <tdevries@suse.de>

	* testsuite/libgomp.oacc-fortran/lib-12.f90: Move acc_async_test calls
	to correct locations.  Remove xfail.

From-SVN: r262990