make arch __clone backends hidden

these are not a public interface and are not intended to be callable
from anywhere but the public clone function or other places in libc.

src/internal/pthread_impl.h

@@ -141,7 +141,7 @@ int __init_tp(void *);
 void *__copy_tls(unsigned char *);
 void __reset_tls();
 
-int __clone(int (*)(void *), void *, int, void *, ...);
+hidden int __clone(int (*)(void *), void *, int, void *, ...);
 int __set_thread_area(void *);
 int __libc_sigaction(int, const struct sigaction *, struct sigaction *);
 void __unmapself(void *, size_t);

src/thread/aarch64/clone.s

@@ -5,6 +5,7 @@
 //         x8,        x0,    x1,    x2,   x3,  x4
 
 .global __clone
+.hidden __clone
 .type   __clone,%function
 __clone:
 	// align stack and save func,arg

src/thread/arm/clone.s

 .syntax unified
 .text
 .global __clone
+.hidden __clone
 .type   __clone,%function
 __clone:
 	stmfd sp!,{r4,r5,r6,r7}

src/thread/i386/clone.s

 .text
 .global __clone
+.hidden __clone
 .type   __clone,@function
 __clone:
 	push %ebp

src/thread/m68k/clone.s

 .text
 .global __clone
+.hidden __clone
 .type   __clone,@function
 __clone:
 	movem.l %d2-%d5,-(%sp)

src/thread/microblaze/clone.s

 .global __clone
+.hidden __clone
 .type   __clone,@function
 
 # r5, r6, r7, r8, r9, r10, stack

src/thread/mips/clone.s

 .set noreorder
 .global __clone
+.hidden __clone
 .type   __clone,@function
 __clone:
 	# Save function pointer and argument pointer on new thread stack

src/thread/mips64/clone.s

 .set	noreorder
 .global	__clone
+.hidden __clone
 .type	__clone,@function
 __clone:
 	# Save function pointer and argument pointer on new thread stack

src/thread/mipsn32/clone.s

 .set	noreorder
 .global	__clone
+.hidden __clone
 .type	__clone,@function
 __clone:
 	# Save function pointer and argument pointer on new thread stack

src/thread/or1k/clone.s

@@ -3,6 +3,7 @@
  * sys_clone(flags, stack, ptid, ctid, tls)
  */
 .global __clone
+.hidden __clone
 .type   __clone,@function
 __clone:
 	l.addi	r4, r4, -8

src/thread/powerpc/clone.s

 .text
 .global __clone
+.hidden __clone
 .type __clone, %function
 __clone:
 # int clone(fn, stack, flags, arg, ptid, tls, ctid)

src/thread/powerpc64/clone.s

 .text
 .global __clone
+.hidden __clone
 .type __clone, %function
 __clone:
 	# int clone(fn, stack, flags, arg, ptid, tls, ctid)

src/thread/s390x/clone.s

 .text
 .global __clone
+.hidden __clone
 .type __clone, %function
 __clone:
 	# int clone(

src/thread/sh/clone.s

 .text
 .global __clone
+.hidden __clone
 .type   __clone, @function
 __clone:
 ! incoming: fn stack flags arg ptid tls      ctid

src/thread/x32/clone.s

 .text
 .global __clone
+.hidden __clone
 .type   __clone,@function
 __clone:
 	movl $0x40000038,%eax /* SYS_clone */

src/thread/x86_64/clone.s

 .text
 .global __clone
+.hidden __clone
 .type   __clone,@function
 __clone:
 	xor %eax,%eax