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: 33c9da5 ("um: Rewrite the sigio workaround based on epoll and tgkill")
Signed-off-by: Yongting Lin <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Johannes Berg <[email protected]>

Author: ivanalgo

arch/um/os-Linux/sigio.c

@@ -12,6 +12,7 @@
 #include <signal.h>
 #include <string.h>
 #include <sys/epoll.h>
+#include <asm/unistd.h>
 #include <kern_util.h>
 #include <init.h>
 #include <os.h>
@@ -46,7 +47,7 @@ static void *write_sigio_thread(void *unused)
 			       __func__, errno);
 		}
 
-		CATCH_EINTR(r = tgkill(pid, pid, SIGIO));
+		CATCH_EINTR(r = syscall(__NR_tgkill, pid, pid, SIGIO));
 		if (r < 0)
 			printk(UM_KERN_ERR "%s: tgkill failed, errno = %d\n",
 			       __func__, errno);