libc: add <sys/eventfd.h> and corresponding implementations.

Change-Id: Ide040884c456190226e580513099fdb8377e015b
diff --git a/libc/include/sys/linux-syscalls.h b/libc/include/sys/linux-syscalls.h
index b90f20d..9702a7a 100644
--- a/libc/include/sys/linux-syscalls.h
+++ b/libc/include/sys/linux-syscalls.h
@@ -182,6 +182,7 @@
 #define __NR_inotify_init                 (__NR_SYSCALL_BASE + 316)
 #define __NR_inotify_add_watch            (__NR_SYSCALL_BASE + 317)
 #define __NR_inotify_rm_watch             (__NR_SYSCALL_BASE + 318)
+#define __NR_eventfd2                     (__NR_SYSCALL_BASE + 356)
 #define __NR_ARM_set_tls                  (__NR_SYSCALL_BASE + 983045)
 #define __NR_ARM_cacheflush               (__NR_SYSCALL_BASE + 983042)
 #endif
@@ -225,6 +226,7 @@
 #define __NR_inotify_init                 (__NR_SYSCALL_BASE + 291)
 #define __NR_inotify_add_watch            (__NR_SYSCALL_BASE + 292)
 #define __NR_inotify_rm_watch             (__NR_SYSCALL_BASE + 293)
+#define __NR_eventfd2                     (__NR_SYSCALL_BASE + 328)
 #endif
 
 #if defined(__SH3__) || defined(__SH4__) 
@@ -282,6 +284,7 @@
 #define __NR_inotify_init                 (__NR_SYSCALL_BASE + 290)
 #define __NR_inotify_add_watch            (__NR_SYSCALL_BASE + 291)
 #define __NR_inotify_rm_watch             (__NR_SYSCALL_BASE + 292)
+#define __NR_eventfd2                     (__NR_SYSCALL_BASE + 328)
 #endif
 
 #endif