Fix GNU C and Fortran flags on SunOS

The commit "Split GNU compiler information files" intended to move GNU
flags from the platform-wide Platform/SunOS.cmake module into

  Platform/SunOS-GNU-<lang>.cmake

using a helper module Platform/SunOS-GNU.cmake to consolidate flags.
However, it accidentally put Fortran flags in the C language module and
left out the Fortran module altogether.  This fixes those mistakes.

Modules/Platform/SunOS-GNU-C.cmake

@@ -1,2 +1,2 @@
 include(Platform/SunOS-GNU)
-__sunos_compiler_gnu(Fortran)
+__sunos_compiler_gnu(C)

Modules/Platform/SunOS-GNU-Fortran.cmake

@@ -0,0 +1,2 @@
+include(Platform/SunOS-GNU)
+__sunos_compiler_gnu(Fortran)