Reinstate kernels-restrict behaviour

Reinstate kernels-restrict behaviour

This patch contains a small fix for upstream churn relative to the last version
posted.

2018-09-20  Cesar Philippidis  <cesar@codesourcery.com>
	    Julian Brown  <julian@codesourcery.com>

	* omp-low.c (install_var_field): New base_pointer_restrict
	argument.
	(scan_sharing_clauses): Update call to install_var_field.
	(omp_target_base_pointers_restrict_p): New function.
	(scan_omp_target): Update call to install_var_field.

(cherry picked from openacc-gcc-9-branch commit
99a4260fe264624136eda666363464eba48bdc3e)