Support for building and running on Perlmutter
I'm hitting an issue that prevents me from getting a job at the moment, but this should build and hypothetically run on Perlmutter.
Summary of changes:
- Add UCX submodule (required by GASNet)
- Update GASNet submodule
- Update Legion submodule
- Update build detection for Perlmutter (keys off of
NERSC_HOST
since the (even fully-qualified) hostname doesn't contain the information we need)