Wrap remaining extern symbols in .c files into EXTERN_C_BEGIN/END

Wrap remaining extern symbols in .c files into EXTERN_C_BEGIN/END

Issue #206 (bdwgc).

* dyn_load.c [SOLARISDL && !USE_PROC_FOR_LIBRARIES] (_DYNAMIC):
Move the extern symbol declaration out of a function body.
* os_dep.c [!PCR && !MACOS && REDIRECT_MALLOC && GC_SOLARIS_THREADS]
(sbrk): Likewise.
* dyn_load.c [SOLARISDL && !USE_PROC_FOR_LIBRARIES] (_DYNAMIC):
Wrap into EXTERN_C_BEGIN/END block.
* dyn_load.c [__ELF__ && LINUX && HOST_ANDROID || HAVE_DL_ITERATE_PHDR]
(dl_iterate_phdr): Likewise.
* dyn_load.c [!USE_PROC_FOR_LIBRARIES] (_DYNAMIC): Likewise.
* dyn_load.c [ALPHA && OSF1 || HPUX] (sys_errlist, sys_nerr, errno):
Likewise.
* os_dep.c [SEARCH_FOR_DATA_START && (LINUX || HURD)] (__data_start,
data_start): Likewise.
* os_dep.c [SEARCH_FOR_DATA_START && LINUX && HOST_ANDROID] (_etext,
__dso_handle): Likewise.
* os_dep.c [NETBSD && __ELF__] (environ): Likewise.
* os_dep.c [OPENBSD && THREADS] (__syscall): Likewise.
* os_dep.c [LINUX_STACKBOTTOM && USE_LIBC_PRIVATES] (__libc_stack_end):
Likewise.
* os_dep.c [LINUX_STACKBOTTOM && USE_LIBC_PRIVATES && IA64]
(__libc_ia64_register_backing_store_base): Likewise.
* os_dep.c [SYMBIAN] (GC_get_main_symbian_stack_base): Likewise.
* os_dep.c [!PCR && !MACOS && REDIRECT_MALLOC && GC_SOLARIS_THREADS]
(sbrk): Likewise.
* os_dep.c [MMAP_SUPPORTED && SYMBIAN]
(GC_get_private_path_and_zero_file): Likewise.
* pthread_stop_world.c [USE_TKILL_ON_ANDROID] (tkill): Likewise.
* pthread_stop_world.c [NACL] (nacl_interface_query): Likewise.
* pthread_support.c [PTHREAD_REGISTER_CANCEL_WEAK_STUBS]
(__pthread_register_cancel, __pthread_unregister_cancel): Likewise.
* pthread_support.c [PTHREAD_REGISTER_CANCEL_WEAK_STUBS]
(__pthread_register_cancel, __pthread_unregister_cancel): Use "void"
to denote empty arguments list.