Build flag to keep nvhpc GPU builds bit-identical with CPU builds

[abishekg7]

In order for MPAS-A OpenACC runs on GPU to remain bit-identical with the CPU runs, we need to use the -Mnofma ( no Fuse-Multiply-Add) switch to the NVHPC CPU build and -Mnofma -gpu=math_uniform for the NVHPC GPU builds. This PR introduces a new compile-time switch NVHPC_BFB to request the CPU and GPU builds have these properties.

Prior to this PR, the -Mnofma was only set inside FFLAGS_ACC, which did nevertheless not apply to CPU builds. This has been fixed. Additionally, with this change, we could potentially remove the -Mnofma option from the optimized builds, and only selectively request it for verifying the GPU results against the CPU reference.

The usage is as follows:

CPU build:

make nvhpc -j4 NVHPC_BFB=true

GPU build:

make nvhpc -j4 OPENACC=true NVHPC_BFB=true