[OpenACC] Refactor 'present_create_copy' into 'goacc_enter_data'

Every caller passes in 'FLAG_PRESENT', 'FLAG_CREATE'.  Change the remaining
'FLAG_COPY' into the usual map kind.

No functional changes.

	libgomp/
	* oacc-mem.c (present_create_copy): Refactor into...
	(goacc_enter_data): ... this.  Adjust all users.

From-SVN: r279535