[og9] Target-dependent gang-private variable decl rewriting

[og9] Target-dependent gang-private variable decl rewriting

	gcc/
	* omp-offload.c (convert.h): Include.
	(struct addr_expr_rewrite_info): Add struct.
	(rewrite_addr_expr): New function.
	(is_sync_builtin_call): New function.
	(execute_oacc_device_lower): Support rewriting gang-private variables
	using target hook, and fix up addr_expr nodes afterwards.
	* target.def (adjust_gangprivate_decl): New target hook.
	* doc/tm.texi.in (TARGET_GOACC_ADJUST_GANGPRIVATE_DECL): Document new
	target hook.
	* doc/tm.texi: Regenerate.

(cherry picked from openacc-gcc-9-branch commit
2c432092fae99930879687f88f2e8e97d29c786d)