um: Do not rely on libc to provide modify_ldt()

modify_ldt() was declared as an external symbol. Despite the man
page for this syscall telling that there is no wrapper in glibc,
since version 2.1 there actually is, so linking to the glibc
works.

Since modify_ldt() is not a POSIX interface, other libc
implementations do not always provide a wrapper function.
Even glibc headers do not provide a corresponding declaration.

So go the recommended way to call this using syscall().
Signed-off-by: NHans-Werner Hilse <hwhilse@gmail.com>
Signed-off-by: NRichard Weinberger <richard@nod.at>

arch/x86/um/ldt.c

@@ -12,7 +12,10 @@
 #include <skas.h>
 #include <sysdep/tls.h>
 
-extern int modify_ldt(int func, void *ptr, unsigned long bytecount);
+static inline int modify_ldt (int func, void *ptr, unsigned long bytecount)
+{
+	return syscall(__NR_modify_ldt, func, ptr, bytecount);
+}
 
 static long write_ldt_entry(struct mm_id *mm_idp, int func,
 		     struct user_desc *desc, void **addr, int done)