microblaze: Add TLS support to sys_clone

Formerly unused Args 4/5 now load parent tid / child tid so the brid to
do_fork can pick up TLS from r10. Arg 3 still unused

There is also necessary to fix old glibc which do not setup r9/r10 (arg 4/5).
Simple clearing them is fine.
Signed-off-by: NEdgar E. Iglesias <edgar.iglesias@gmail.com>
Signed-off-by: NDavid Holsgrove <david.holsgrove@petalogix.com>
Signed-off-by: NMichal Simek <monstr@monstr.eu>

arch/microblaze/kernel/entry.S

@@ -493,10 +493,11 @@ C_ENTRY(sys_clone):
 	bnei	r6, 1f;			/* See if child SP arg (arg 1) is 0. */
 	lwi	r6, r1, PT_R1;	/* If so, use paret's stack ptr */
 1:	addik	r7, r1, 0;			/* Arg 2: parent context */
-	add	r8, r0, r0;			/* Arg 3: (unused) */
-	add	r9, r0, r0;			/* Arg 4: (unused) */
+	lwi     r9, r1, PT_R8;          /* parent tid.  */
+	lwi     r10, r1, PT_R9;         /* child tid.  */
+	/* do_fork will pick up TLS from regs->r10.  */
 	brid	do_fork		/* Do real work (tail-call) */
-	add	r10, r0, r0;			/* Arg 5: (unused) */
+	add     r8, r0, r0;             /* Arg 3: (unused) */
 
 C_ENTRY(sys_execve):
 	brid	microblaze_execve;	/* Do real work (tail-call).*/