arm: Use _dl_find_object on __gnu_Unwind_Find_exidx (BZ 31405)

Instead of __dl_iterate_phdr. On ARM dlfo_eh_frame/dlfo_eh_count
maps to PT_ARM_EXIDX vaddr start / length.

On a Neoverse N1 machine with 160 cores, the following program:

  $ cat test.c
  #include <stdlib.h>
  #include <pthread.h>
  #include <assert.h>

  enum {
    niter = 1024,
    ntimes = 128,
  };

  static void *
  tf (void *arg)
  {
    int a = (int) arg;

    for (int i = 0; i < niter; i++)
      {
        void *p[ntimes];
        for (int j = 0; j < ntimes; j++)
  	p[j] = malloc (a * 128);
        for (int j = 0; j < ntimes; j++)
  	free (p[j]);
      }

    return NULL;
  }

  int main (int argc, char *argv[])
  {
    enum { nthreads = 16 };
    pthread_t t[nthreads];

    for (int i = 0; i < nthreads; i ++)
      assert (pthread_create (&t[i], NULL, tf, (void *) i) == 0);

    for (int i = 0; i < nthreads; i++)
      {
        void *r;
        assert (pthread_join (t[i], &r) == 0);
        assert (r == NULL);
      }

    return 0;
  }
  $ arm-linux-gnueabihf-gcc -fsanitize=address test.c -o test

Improves from ~15s to 0.5s.

Checked on arm-linux-gnueabihf.

elf/dl-find_object.c

@@ -356,7 +356,7 @@ _dlfo_lookup (uintptr_t pc, struct dl_find_object_internal *first1, size_t size)
 }
 
 int
-_dl_find_object (void *pc1, struct dl_find_object *result)
+__dl_find_object (void *pc1, struct dl_find_object *result)
 {
   uintptr_t pc = (uintptr_t) pc1;
 
@@ -463,7 +463,8 @@ _dl_find_object (void *pc1, struct dl_find_object *result)
         return -1;
     } /* Transaction retry loop.  */
 }
-rtld_hidden_def (_dl_find_object)
+hidden_def (__dl_find_object)
+weak_alias (__dl_find_object, _dl_find_object)
 
 /* _dlfo_process_initial is called twice.  First to compute the array
    sizes from the initial loaded mappings.  Second to fill in the