GASNet: Too many simultaneous local client threads

[elliottslaughter]

In some configurations, GASNet may generate the following error:

GASNet Extended API: Too many simultaneous local client threads (limit=256). To raise this limit, configure GASNet using --with-max-pthreads-per-node=N

This happens when too many threads attempt to access GASNet at once. It is especially likely to happen if the user also passes -ll:force_kthreads, but it can (in principle) happen without it as well.

The workaround for this issue is to set -gex:immediate 0.

I'm not aware of whether a permanent solution to this issue is planned or not, but at the very least we need to document the workaround.

Split from #1508 (comment)