Kernel/LibC: add mmap for private anonymous mappings

This will be used by the userspace to get more memory. Currently
kernel handles all allocations, which is not preferable.

libc/sys/mman.cpp

@@ -0,0 +1,23 @@
+#include <sys/mman.h>
+#include <sys/syscall.h>
+#include <unistd.h>
+
+void* mmap(void* addr, size_t len, int prot, int flags, int fildes, off_t off)
+{
+	sys_mmap_t args {
+		.addr = addr,
+		.len = len,
+		.prot = prot,
+		.flags = flags,
+		.off = off
+	};
+	long ret = syscall(SYS_MMAP, &args);
+	if (ret == -1)
+		return nullptr;
+	return (void*)ret;
+}
+
+int munmap(void* addr, size_t len)
+{
+	return syscall(SYS_MUNMAP, addr, len);
+}