Change most internal uses of time to __clock_gettime.

As for gettimeofday, time will be implemented based on clock_gettime
on all platforms and internal code should use clock_gettime
directly.  In addition to removing a layer of indirection, this will
allow us to remove the PLT-bypass gunk for gettimeofday.

The changed code always assumes __clock_gettime (CLOCK_REALTIME)
or __clock_gettime (CLOCK_REALTIME_COARSE) (for Linux case) cannot
fail, using the same rationale for gettimeofday change.  And internal
helper was added (time_now).

Checked on x86_64-linux-gnu, i686-linux-gnu, powerpc64le-linux-gnu,
powerpc64-linux-gnu, and powerpc-linux-gnu.

Reviewed-by: Lukasz Majewski <lukma@denx.de>

sysdeps/unix/sysv/linux/i386/time.c

@@ -1,34 +0,0 @@
-/* time -- Get number of seconds since Epoch.  Linux/i386 version.
-   Copyright (C) 2015-2019 Free Software Foundation, Inc.
-   This file is part of the GNU C Library.
-
-   The GNU C Library is free software; you can redistribute it and/or
-   modify it under the terms of the GNU Lesser General Public
-   License as published by the Free Software Foundation; either
-   version 2.1 of the License, or (at your option) any later version.
-
-   The GNU C Library is distributed in the hope that it will be useful,
-   but WITHOUT ANY WARRANTY; without even the implied warranty of
-   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
-   Lesser General Public License for more details.
-
-   You should have received a copy of the GNU Lesser General Public
-   License along with the GNU C Library; if not, see
-   <https://www.gnu.org/licenses/>.  */
-
-#ifdef SHARED
-# define time __redirect_time
-#endif
-
-#include <time.h>
-
-#ifdef SHARED
-# undef time
-# define time_type __redirect_time
-
-# undef libc_hidden_def
-# define libc_hidden_def(name)  \
-  __hidden_ver1 (__time_syscall, __GI_time, __time_syscall);
-#endif
-
-#include <sysdeps/unix/sysv/linux/x86/time.c>