Add GNAT.CPP packages

GNAT.CPP_Exceptions has gained new subprograms that return C++
type_info objects, so we need the new GNAT.CPP units where the Ada
bindings for this type are defined.

Issue: eng/toolchain/gcc#119

support/rts_sources/sources.py

@@ -678,6 +678,13 @@
             "libgnat/g-casuti.adb",
             "libgnat/g-comver.ads",
             "libgnat/g-comver.adb",
+            "libgnat/g-cpp.ads",
+            "libgnat/g-cppexc.ads",
+            "libgnat/g-cppexc.adb",
+            "libgnat/g-cppstd.ads",
+            "libgnat/g-cppstd.adb",
+            "libgnat/g-cstyin.ads",
+            "libgnat/g-cstyin.adb",
             "libgnat/g-crc32.ads",
             "libgnat/g-crc32.adb",
             "libgnat/g-dyntab.ads",