Remove Fortran only flag from LDFLAGS_DEBUG for NVIDIA compiler targets

[gdicker1]

The -Mchkptr flag is only valid for Fortran compilation. If included in a C build with NVHPC compilers (like the check with openacc.c does), the build fails.

Before this change, builds using the nvhpc target and OPENACC=true and DEBUG=true would fail. Removing -Mchkptr allows successful debug openacc builds.

[gdicker1]

The build of openacc.c occurs as part of the openacc_test target in the top-level Makefile. Without this change, builds fail around line 1123 of the top-level Makefile (before any actual MPAS files are touched).

User's would then have to compile the openacc.c program (as directed by the build) to see the error message related to -Mchkptr.

[mgduda]

@gdicker1 Just a note for similar PRs in future, since the changes are in the top-level Makefile and not specific to MPAS-Atmosphere a branch name with framework/ rather than atmosphere/ may be preferable. But no worries for this PR!

[gdicker1]

@gdicker1 Just a note for similar PRs in future, since the changes are in the top-level Makefile and not specific to MPAS-Atmosphere a branch name with framework/ rather than atmosphere/ may be preferable. But no worries for this PR!

Thanks for the reminder!