Add a restriction on allocate clause (OpenMP 5.0)

Add a restriction on allocate clause (OpenMP 5.0)

An allocate clause in target region must specify an allocator
unless the compilation unit has requires construct with
dynamic_allocators clause.  Current implementation of the allocate
clause did not check for this restriction. This patch fills that
gap.

gcc/ChangeLog:

	* omp-low.cc (omp_maybe_offloaded_ctx): New prototype.
	(scan_sharing_clauses):  Check a restriction on allocate clause.

gcc/testsuite/ChangeLog:

	* c-c++-common/gomp/allocate-2.c: Add tests.
	* c-c++-common/gomp/allocate-8.c: New test.
	* gfortran.dg/gomp/allocate-3.f90: Add tests.
	* gcc.dg/gomp/pr104517.c: Update.