Move Symbian data-roots-related files to 'extra/symbian' folder

* extra/init_global_static_roots.cpp: Move to "extra/symbian" folder.
* extra/global_start.cpp: Likewise.
* extra/global_end.cpp: Likewise.
* Makefile.am (EXTRA_DIST): Update references to moved Symbian files.
* Makefile.direct (OTHER_MAKEFILES): Likewise.

Makefile.am

@@ -179,8 +179,8 @@ EXTRA_DIST += BCC_MAKEFILE NT_MAKEFILE \
 EXTRA_DIST += tools/add_gc_prefix.c tools/gcname.c tools/if_mach.c \
     tools/if_not_there.c tools/setjmp_t.c tools/threadlibs.c \
     gc.mak extra/MacOS.c extra/AmigaOS.c \
-    extra/global_end.cpp extra/global_start.cpp \
-    extra/init_global_static_roots.cpp extra/symbian.cpp \
+    extra/symbian/global_end.cpp extra/symbian/global_start.cpp \
+    extra/symbian/init_global_static_roots.cpp extra/symbian.cpp \
     build/s60v3/bld.inf build/s60v3/libgc.mmp \
     extra/Mac_files/datastart.c extra/Mac_files/dataend.c \
     extra/Mac_files/MacOS_config.h \

Makefile.direct

@@ -144,8 +144,8 @@ OTHER_MAKEFILES= OS2_MAKEFILE NT_MAKEFILE gc.mak \
 OTHER_FILES= tools/setjmp_t.c tools/callprocs.sh extra/MacOS.c \
   extra/Mac_files/datastart.c extra/Mac_files/dataend.c \
   extra/Mac_files/MacOS_config.h tools/add_gc_prefix.c gc_cpp.cpp \
-  extra/global_end.cpp extra/global_start.cpp \
-  extra/init_global_static_roots.cpp extra/symbian.cpp \
+  extra/symbian/global_end.cpp extra/symbian/global_start.cpp \
+  extra/symbian/init_global_static_roots.cpp extra/symbian.cpp \
   build/s60v3/bld.inf build/s60v3/libgc.mmp \
   extra/AmigaOS.c extra/msvc_dbg.c include/private/msvc_dbg.h \
   $(TESTS) $(GNU_BUILD_FILES) $(OTHER_MAKEFILES)