Add accept4() syscall

Add accept4() using SYSCALLS.TXT and gensyscall

Change-Id: I6f19f29144186d15d46423e10f2cc4b4223719c6
diff --git a/libc/SYSCALLS.TXT b/libc/SYSCALLS.TXT
index 8d4b258..e597b2f 100644
--- a/libc/SYSCALLS.TXT
+++ b/libc/SYSCALLS.TXT
@@ -238,6 +238,7 @@
 int           connect(int, struct sockaddr*, socklen_t)   arm,arm64,mips,mips64,x86_64
 int           listen(int, int)                   arm,arm64,mips,mips64,x86_64
 int           accept(int, struct sockaddr*, socklen_t*)  arm,arm64,mips,mips64,x86_64
+int           accept4(int, struct sockaddr*, socklen_t*, int)  arm,arm64,mips,mips64,x86_64
 int           getsockname(int, struct sockaddr*, socklen_t*)  arm,arm64,mips,mips64,x86_64
 int           getpeername(int, struct sockaddr*, socklen_t*)  arm,arm64,mips,mips64,x86_64
 int           sendto(int, const void*, size_t, int, const struct sockaddr*, socklen_t)  arm,arm64,mips,mips64,x86_64
@@ -266,6 +267,7 @@
 int           getsockopt:socketcall:15(int, int, int, void*, socklen_t*)    x86
 int           sendmsg:socketcall:16(int, const struct msghdr*, unsigned int)  x86
 int           recvmsg:socketcall:17(int, struct msghdr*, unsigned int)   x86
+int           accept4:socketcall:18(int, struct sockaddr*, socklen_t*, int)  x86
 int           recvmmsg:socketcall:19(int, struct mmsghdr*, unsigned int, int, const struct timespec*)   x86
 int           sendmmsg:socketcall:20(int, struct mmsghdr*, unsigned int, int)   x86