um: Fix tgkill compile error on old host OSes

um: Fix tgkill compile error on old host OSes

tgkill is a quite old syscall since kernel 2.5.75, but unfortunately glibc
doesn't support it before 2.30. Thus some systems fail to compile the
latest UserMode Linux.

Here is the compile error I encountered when I tried to compile UML in
my system shipped with glibc-2.28.

    CALL    scripts/checksyscalls.sh
    CC      arch/um/os-Linux/sigio.o
  In file included from arch/um/os-Linux/sigio.c:17:
  arch/um/os-Linux/sigio.c: In function ‘write_sigio_thread’:
  arch/um/os-Linux/sigio.c:49:19: error: implicit declaration of function ‘tgkill’; did you mean ‘kill’? [-Werror=implicit-function-declaration]
     CATCH_EINTR(r = tgkill(pid, pid, SIGIO));
                     ^~~~~~
  ./arch/um/include/shared/os.h:21:48: note: in definition of macro ‘CATCH_EINTR’
  #define CATCH_EINTR(expr) while ((errno = 0, ((expr) < 0)) && (errno == EINTR))
                                                ^~~~
  cc1: some warnings being treated as errors

Fix it by Replacing glibc call with raw syscall.

Fixes: 33c9da5dfb18 ("um: Rewrite the sigio workaround based on epoll and tgkill")
Signed-off-by: Yongting Lin <linyongting@gmail.com>
Link: https://patch.msgid.link/20250527151222.40371-1-linyongting@gmail.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>