make arch __set_thread_area backends hidden

this is not a public interface, and does not even necessarily match
the syscall on all archs that have a syscall by that name.

on archs where it's implemented in C, no action on the source file is
needed; the hidden declaration in pthread_arch.h suffices.

src/internal/pthread_impl.h

@@ -142,7 +142,7 @@ void *__copy_tls(unsigned char *);
 void __reset_tls();
 
 hidden int __clone(int (*)(void *), void *, int, void *, ...);
-int __set_thread_area(void *);
+hidden int __set_thread_area(void *);
 int __libc_sigaction(int, const struct sigaction *, struct sigaction *);
 void __unmapself(void *, size_t);
 

src/thread/aarch64/__set_thread_area.s

@@ -1,4 +1,5 @@
 .global __set_thread_area
+.hidden __set_thread_area
 .type   __set_thread_area,@function
 __set_thread_area:
 	msr tpidr_el0,x0

src/thread/i386/__set_thread_area.s

@@ -1,5 +1,6 @@
 .text
 .global __set_thread_area
+.hidden __set_thread_area
 .type   __set_thread_area,@function
 __set_thread_area:
 	push %ebx

src/thread/microblaze/__set_thread_area.s

@@ -1,4 +1,5 @@
 .global __set_thread_area
+.hidden __set_thread_area
 .type   __set_thread_area,@function
 __set_thread_area:
 	ori      r21, r5, 0

src/thread/or1k/__set_thread_area.s

@@ -1,4 +1,5 @@
 .global __set_thread_area
+.hidden __set_thread_area
 .type   __set_thread_area,@function
 __set_thread_area:
 	l.ori	r10, r3, 0

src/thread/powerpc/__set_thread_area.s

@@ -1,5 +1,6 @@
 .text
 .global __set_thread_area
+.hidden __set_thread_area
 .type   __set_thread_area, %function
 __set_thread_area:
 	# mov pointer in reg3 into r2

src/thread/powerpc64/__set_thread_area.s

@@ -1,5 +1,6 @@
 .text
 .global __set_thread_area
+.hidden __set_thread_area
 .type   __set_thread_area, %function
 __set_thread_area:
 	mr 13, 3

src/thread/s390x/__set_thread_area.s

@@ -1,5 +1,6 @@
 .text
 .global __set_thread_area
+.hidden __set_thread_area
 .type   __set_thread_area, %function
 __set_thread_area:
 	sar  %a1, %r2

src/thread/x32/__set_thread_area.s

@@ -1,6 +1,7 @@
 /* Copyright 2011-2012 Nicholas J. Kain, licensed under standard MIT license */
 .text
 .global __set_thread_area
+.hidden __set_thread_area
 .type __set_thread_area,@function
 __set_thread_area:
 	mov %edi,%esi           /* shift for syscall */

src/thread/x86_64/__set_thread_area.s

@@ -1,6 +1,7 @@
 /* Copyright 2011-2012 Nicholas J. Kain, licensed under standard MIT license */
 .text
 .global __set_thread_area
+.hidden __set_thread_area
 .type __set_thread_area,@function
 __set_thread_area:
 	mov %rdi,%rsi           /* shift for syscall */