[gomp] openacc reduction cleanup

[Cesar Philippidis]

This patch removes a lot of the crufty code which was necessary for an
arbitrary sized vector reductions. The new plan going forward it to fix
vector_length to a size such that vector loops don't require any
synchronization after the loop have terminated. In the case of nvptx
targets, vector_length = warp_sz, which is currently 32 threads. I'll
follow up this patch with another one which adds support for tree
reductions in vector loops in a later date.

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

Cesar