fix: replace fragmented bc member GPU_UPDATE with single whole-struct update

Compilers (nvfortran/CCE with OpenMP target, and AMD amdflang) fail or
generate incorrect device pointer arithmetic when target update uses
3-level-deep struct member paths like bc%y%beg_side%vel_wall.

bc_xyz_info has no allocatable/pointer members, so updating the entire
bc struct atomically is safe and generates ! target update to(bc) /
! update device(bc) — one directive vs 14 fragmented member paths.

Author: sbryngelson

src/simulation/m_start_up.fpp

@@ -1080,27 +1080,14 @@ contains
         $:GPU_UPDATE(device='[acoustic_source, num_source]')
         $:GPU_UPDATE(device='[sigma, surface_tension]')
 
-        $:GPU_UPDATE(device='[dx, x_cc, x_cb]')
+        $:GPU_UPDATE(device='[x%spacing, x%cb, x%cc]')
         if (n > 0) then
-            $:GPU_UPDATE(device='[dy, y_cc, y_cb]')
+            $:GPU_UPDATE(device='[y%spacing, y%cb, y%cc]')
         end if
         if (p > 0) then
-            $:GPU_UPDATE(device='[dz, z_cc, z_cb]')
+            $:GPU_UPDATE(device='[z%spacing, z%cb, z%cc]')
         end if
-        $:GPU_UPDATE(device='[bc%x%beg, bc%x%end, bc%y%beg, bc%y%end, bc%z%beg, bc%z%end]')
-        $:GPU_UPDATE(device='[bc%x%beg_side%vel_wall, bc%x%end_side%vel_wall]')
-        $:GPU_UPDATE(device='[bc%y%beg_side%vel_wall, bc%y%end_side%vel_wall]')
-        $:GPU_UPDATE(device='[bc%z%beg_side%vel_wall, bc%z%end_side%vel_wall]')
-
-        $:GPU_UPDATE(device='[bc%x%beg_side%grcbc, bc%x%end_side%grcbc, bc%x%end_side%grcbc_vel]')
-        $:GPU_UPDATE(device='[bc%y%beg_side%grcbc, bc%y%end_side%grcbc, bc%y%end_side%grcbc_vel]')
-        $:GPU_UPDATE(device='[bc%z%beg_side%grcbc, bc%z%end_side%grcbc, bc%z%end_side%grcbc_vel]')
-
-        $:GPU_UPDATE(device='[bc%x%beg_side%isothermal, bc%x%end_side%isothermal]')
-        $:GPU_UPDATE(device='[bc%y%beg_side%isothermal, bc%y%end_side%isothermal]')
-        $:GPU_UPDATE(device='[bc%z%beg_side%isothermal, bc%z%end_side%isothermal]')
-        $:GPU_UPDATE(device='[bc%x%beg_side%T_wall, bc%x%end_side%T_wall, bc%y%beg_side%T_wall, bc%y%end_side%T_wall, &
-                     & bc%z%beg_side%T_wall, bc%z%end_side%T_wall]')
+        $:GPU_UPDATE(device='[bc]')
 
         $:GPU_UPDATE(device='[relax, relax_model]')
         if (relax) then