[OpenACC] Refactor 'goacc_enter_data' so that it can be called from...

[OpenACC] Refactor 'goacc_enter_data' so that it can be called from 'goacc_insert_pointer', "present" case, and simplify

No functional changes.

	libgomp/
	* oacc-mem.c (goacc_enter_data): Refactor, so that it can be
	called...
	(goacc_insert_pointer): ... from here, "present" case.
	(goacc_insert_pointer): Inline function into...
	(GOACC_enter_exit_data): ... here, and simplify.

From-SVN: r279540