Add tgkill syscall.

Use tgkill instead of tkill to implement pthread_kill.
This is safer in the event that the thread has already terminated
and its id has been reused by a different process.

Change-Id: Ied715e11d7eadeceead79f33db5e2b5722954ac9
diff --git a/libc/include/sys/linux-syscalls.h b/libc/include/sys/linux-syscalls.h
index 7b74a4b..930508a 100644
--- a/libc/include/sys/linux-syscalls.h
+++ b/libc/include/sys/linux-syscalls.h
@@ -198,6 +198,7 @@
 #define __NR_waitid                       (__NR_SYSCALL_BASE + 284)
 #define __NR_kill                         (__NR_SYSCALL_BASE + 37)
 #define __NR_tkill                        (__NR_SYSCALL_BASE + 238)
+#define __NR_tgkill                       (__NR_SYSCALL_BASE + 270)
 #define __NR_set_thread_area              (__NR_SYSCALL_BASE + 243)
 #define __NR_openat                       (__NR_SYSCALL_BASE + 295)
 #define __NR_madvise                      (__NR_SYSCALL_BASE + 219)
@@ -242,6 +243,7 @@
 #define __NR_waitid                       (__NR_SYSCALL_BASE + 284)
 #define __NR_kill                         (__NR_SYSCALL_BASE + 37)
 #define __NR_tkill                        (__NR_SYSCALL_BASE + 238)
+#define __NR_tgkill                       (__NR_SYSCALL_BASE + 270)
 #define __NR_set_thread_area              (__NR_SYSCALL_BASE + 243)
 #define __NR_vfork                        (__NR_SYSCALL_BASE + 190)
 #define __NR_openat                       (__NR_SYSCALL_BASE + 295)