Optimize ACC port of atm_advance_scalars_work:4760

[abishekg7]

This PR introduces optimizations for the OpenACC port of atm_advance_scalars_work:4760.

The table below lists the timings for a real, global 30km experiment on A100 GPU with nvhpc, and Derecho CPUs with several compiler suites.

Version	GPU kernel time (ms)	CPU timer - nvhpc	CPU timer - gnu		CPU timer - intel25
base	22.569	0.02903	0.03156		0.03324
1 In case default, swap the seq and vector collapse(2) loops and fuse horiz_flux_arr	18	0.02908	0.03318		0.03389
2 Remove ica loop, directly subst advCellsForEdge	16.832	0.03088	0.03248		0.03351
3 case 10, remove separate scalar_weight2 loop and inline it in next loop	12.023	0.03459	0.03204		0.0324
4 local_sign	13.83	0.03459	0.032		0.03239

For nvhpc gpu runs, -gpu=math_uniform is introduced as a build flag to ensure optimizations are bit-identical, and the we report the numbers using NV_ACC_TIME=1. The GPU runs are on 1 Derecho GPU node, using 1 A100 via 1 MPI task.

The numbers reported for the CPU runs with gnu and intel compilers use the newly-added timers local to this region, and are averaged across three runs. The CPU runs use a single derecho CPU node each, fully subscribed to 128 MPI tasks.

This work was completed in part at the NCAR/NLR/NOAA Open Hackathon, part of the Open Hackathons program. The authors would like to acknowledge OpenACC-Standard.org for their support.