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/Android.mk b/libc/Android.mk
index 49c8731..410f242 100644
--- a/libc/Android.mk
+++ b/libc/Android.mk
@@ -353,6 +353,7 @@
 	arch-arm/bionic/kill.S \
 	arch-arm/bionic/libgcc_compat.c \
 	arch-arm/bionic/tkill.S \
+	arch-arm/bionic/tgkill.S \
 	arch-arm/bionic/memcmp.S \
 	arch-arm/bionic/memcmp16.S \
 	arch-arm/bionic/memcpy.S \