Fix shell C kernel symbol names for Fortran linkage (fderivs_sh_ etc.)

Shell C functions must export Fortran-compatible symbols with trailing
underscore so bssn_rhs_ss.f90 and getnp4.f90 can link when WithShell is
active and USE_CXX_SHELL_KERNELS=1 replaces Fortran diff_new_sh.o.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

AMSS_NCKU_source/fdderivs_sh_c.C

@@ -5,7 +5,7 @@
  * fdderivs_sh — second derivatives on shell patch in (rho, sigma, R) coords.
  * Same stencil coefficients as Cartesian fdderivs. Uses symmetry_stbd.
  */
-void fdderivs_sh(const int ex[3],
+extern "C" void fdderivs_sh_(const int ex[3],
                  const double *f,
                  double *fxx, double *fxy, double *fxz,
                  double *fyy, double *fyz, double *fzz,