[NaCl SDK]: use standard __BEGIN_DECLS macros in sys/select.h

There is no reason for sys/select.h to depend on
sdk_util headers.  The __BEGIN_DECLS/__END_DECLS
macros are available in all our toolchains.

This was causing various problems in the naclports
repo.

Review URL: https://codereview.chromium.org/539163004

Cr-Commit-Position: refs/heads/master@{#294044}

native_client_sdk/src/libraries/third_party/newlib-extras/sys/select.h

@@ -6,15 +6,13 @@
 #include <sys/types.h>
 #include <time.h>
 
-#include "sdk_util/macros.h"
-
-EXTERN_C_BEGIN
+__BEGIN_DECLS
 
 int select (int __nfds, fd_set *__restrict __readfds,
             fd_set *__restrict __writefds,
             fd_set *__restrict __exceptfds,
             struct timeval *__restrict __timeout) __THROW;
 
-EXTERN_C_END
+__END_DECLS
 
 #endif /* sys/select.h */