[gomp4] use static_nonchunk loop scheduling for non-static acc gang loops

[Cesar Philippidis]

Tom suggested that it would be more efficient to use static_nochunk for
loop scheduling for acc gang loops when the static argument isn't
present. We also decided that gang(static:*) should be scheduled using
static_nochunk too. A chunk_size of 1 is probably too conservative for
gangs running on gpus.

I also updated the nvptx-specific gang loop test case. My first attempt
wasn't scanning all of the values in the array. This corrects that, and
it also teaches that test what to expect for gang(static:*).

This patch has been committed to gomp-4_0-branch.

Cesar
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