NAMD: FIX build +cuda and mpi ~smp (#20778)

* NAMD: FIX build +cuda

Hi,
If I try to compile NAMD with CUDA support, it fails because cannot file the file "{self.arch}.cuda" because it is undet the "arch" folder.

* NAMD: FIX mpi ~smp

Fix `spack install namd ^charmpp backend=mpi ~smp`

var/spack/repos/builtin/packages/namd/package.py

@@ -111,7 +111,7 @@ def _edit_arch_generic(self, spec, prefix):
                 else:
                     optims_opts = {
                         'gcc': m64 + '-O3 -fexpensive-optimizations \
-                                        -ffast-math ' + archopt,
+                                        -ffast-math -lpthread ' + archopt,
                         'intel': '-O2 -ip ' + archopt,
                         'aocc': m64 + '-O3 -ffp-contract=fast \
                                         -ffast-math ' + archopt}
@@ -226,7 +226,7 @@ def edit(self, spec, prefix):
             self._append_option(opts, 'cuda')
             filter_file('^CUDADIR=.*$',
                         'CUDADIR={0}'.format(spec['cuda'].prefix),
-                        self.arch + '.cuda')
+                        join_path('arch', self.arch + '.cuda'))
 
         config = Executable('./config')