use hidden visibility for sh __unmapself backends

src/thread/sh/__unmapself.c

 #include "pthread_impl.h"
 
-void __unmapself_sh_mmu(void *, size_t);
-void __unmapself_sh_nommu(void *, size_t);
+hidden void __unmapself_sh_mmu(void *, size_t);
+hidden void __unmapself_sh_nommu(void *, size_t);
 
 #if !defined(__SH3__) && !defined(__SH4__)
 #define __unmapself __unmapself_sh_nommu

src/thread/sh/__unmapself_mmu.s

 .text
 .global __unmapself_sh_mmu
+.hidden __unmapself_sh_mmu
 .type   __unmapself_sh_mmu, @function
 __unmapself_sh_mmu:
 	mov   #91, r3  ! SYS_munmap