openacc: Shared memory layout optimisation

This patch implements an algorithm to lay out local data-share (LDS)
space.  It currently works for AMD GCN.  At the moment, LDS is used for
three things:

  1. Gang-private variables
  2. Reduction temporaries (accumulators)
  3. Broadcasting for worker partitioning

After the patch is applied, (2) and (3) are placed at preallocated
locations in LDS, and (1) continues to be handled by the backend (as it
is at present prior to this patch being applied). LDS now looks like this:

  +--------------+ (gang-private size + 1024, = 1536)
  | free space   |
  |    ...       |
  | - - - - - - -|
  | worker bcast |
  +--------------+
  | reductions   |
  +--------------+ <<< -mgang-private-size=<number> (def. 512)
  | gang-private |
  |    vars      |
  +--------------+ (32)
  | low LDS vars |
  +--------------+ LDS base

So, gang-private space is fixed at a constant amount at compile time
(which can be increased with a command-line switch if necessary
for some given code...