Allow libgomp 'cbuf' buffering with OpenACC 'async' for 'ephemeral' data

This does *allow*, but under no circumstances is this currently going to be
used: all potentially applicable data is non-'ephemeral', and thus not
considered for 'gomp_coalesce_buf_add' for OpenACC 'async'.  (But a use will
emerge later.)

Follow-up to commit r12-2530-gd88a6951586c7229b25708f4486eaaf4bf4b5bbe
"Don't use libgomp 'cbuf' buffering with OpenACC 'async'", addressing this
TODO comment:

    TODO ... but we could allow CBUF usage for EPHEMERAL data?  (Open question:
    is it more performant to use libgomp CBUF buffering or individual device
    asyncronous copying?)

Ephemeral data is small, and therefore individual device asyncronous copying
does seem dubious -- in particular given that for all those, we'd individually
have to allocate and queue for deallocation a temporary buffer to capture the
ephemeral data.  Instead, just let the 'cbuf' *be* the temporary buffer.

	libgomp/
	* target.c (gomp_copy_host2dev, gomp_map_vars_internal): Allow
	libgomp 'cbuf' buffering with OpenACC 'async' for 'ephemeral'
	data.