GCC doesn't put builtin function symbol references, which are defined in
the shared C library, in the IR symbol table.  As the result, --as-needed
doesn't work with references to builtin functions:

[hjl@gnu-cfl-3 as-needed]$ cat x.c
#include <stdlib.h>

int
main()
{
  abort ();
  return 0;
}
[hjl@gnu-cfl-3 as-needed]$ cat bar.c
#include <stdlib.h>
#include <stdio.h>

void
abort (void)
{
  printf ("PASS\n");
  exit (0);
}
[hjl@gnu-cfl-3 as-needed]$ make
gcc -B./ -g -flto   -c -o x.o x.c
gcc -B./ -g -fPIC   -c -o bar.o bar.c
gcc -B./ -shared -o libbar.so bar.o
gcc -B./ -Wl,--as-needed -o x x.o libbar.so -Wl,-R,.
./x
make: *** [Makefile:12: all] Aborted (core dumped)
[hjl@gnu-cfl-3 as-needed]$