vmlinux.o: warning: objtool: gdb_serial_stub+0x6d8: machine_restart() missing __noreturn in .c/.h or NORETURN() in noreturns.h

[kernel test robot]

tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
head:   94515f3a7d4256a5062176b7d6ed0471938cd51a
commit: 9f2aee8f7d1842be08da860e45265d30dba0d1f7 kbuild: distributed build support for Clang ThinLTO
date:   7 weeks ago
config: loongarch-randconfig-r064-20260719 (https://download.01.org/0day-ci/archive/20260720/202607200359.Zpe4HXfO-lkp@xxxxxxxxx/config)
compiler: clang version 18.1.8 (https://github.com/llvm/llvm-project 3b5b5c1ec4a3095ab096dd780e84d7ab81f3d7ff)
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20260720/202607200359.Zpe4HXfO-lkp@xxxxxxxxx/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Fixes: 9f2aee8f7d18 ("kbuild: distributed build support for Clang ThinLTO")
| Reported-by: kernel test robot <lkp@xxxxxxxxx>
| Closes: https://lore.kernel.org/oe-kbuild-all/202607200359.Zpe4HXfO-lkp@xxxxxxxxx/

All warnings (new ones prefixed by >>):

>> vmlinux.o: warning: objtool: gdb_serial_stub+0x6d8: machine_restart() missing __noreturn in .c/.h or NORETURN() in noreturns.h
   vmlinux.o: warning: objtool: kernel_entry+0xec: start_kernel() missing __noreturn in .c/.h or NORETURN() in noreturns.h

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