gccNGPackages.gfortran: put libquadmath on the link path

`libgfortran.spec`, which the driver reads out of the `libgfortran` directory
the wrapper already names, links `-lquadmath` unconditionally -- and does so
even though `libgfortran` is configured `--disable-libquadmath`, which
governs building the library rather than the reference to it.

Nothing put that library anywhere the driver would look, so linking any
Fortran failed. Not new in 16; 15 has the same spec.

Co-authored-by: Ben Siraphob <bensiraphob@gmail.com>
Assisted-by: Claude Code (Claude Opus 5)

pkgs/development/compilers/gcc/ng/common/default.nix

@@ -187,6 +187,9 @@ makeScopeWithSplicing' {
           ++ [
             "-B${targetGccPackages.libstdcxx}/lib"
             "-B${targetGccPackages.libgfortran}/lib/"
+            # `libgfortran.spec`, which the driver reads from the directory
+            # above, links `-lquadmath` unconditionally.
+            "-B${targetGccPackages.libquadmath}/lib"
           ]
           ++ threadsCflags;
         };