add clock_adjtime, remap_file_pages, and syncfs syscall wrappers

patch by Justin Cormack, with slight modification

src/linux/clock_adjtime.c

@@ -0,0 +1,9 @@
+#define _GNU_SOURCE
+#include <time.h>
+#include <sys/timex.h>
+#include "syscall.h"
+
+int clock_adjtime (clockid_t clock_id, struct timex *utx)
+{
+	return syscall(SYS_clock_adjtime, clock_id, utx);
+}

src/linux/remap_file_pages.c

@@ -0,0 +1,8 @@
+#define _GNU_SOURCE
+#include <sys/mman.h>
+#include "syscall.h"
+
+int remap_file_pages(void *addr, size_t size, int prot, ssize_t pgoff, int flags)
+{
+	return syscall(SYS_remap_file_pages, addr, size, prot, pgoff, flags);
+}

src/linux/syncfs.c

@@ -0,0 +1,8 @@
+#define _GNU_SOURCE
+#include <unistd.h>
+#include "syscall.h"
+
+void syncfs(int fd)
+{
+	__syscall(SYS_syncfs, fd);
+}