Specify how undefined weak symbol should be resolved in executable

["H.J. Lu" <hjl.tools@gmail.com>]

Currently when building executable, undefined weak symbol is resolved
differently, depending on if the relocatable input is compiled with PIC or
not:

hjl@gnu-tools-1 weak-1]$ cat x.c
extern void __attribute__ ((weak)) fun (void);
int main (void)
{
  if (&fun != 0)
    fun ();
  return 0;
}
[hjl@gnu-tools-1 weak-1]$ make
gcc -B./ -O2 -o x x.c
gcc -B./ -O2 -o y x.c -fPIC
readelf -r x

Relocation section '.rela.dyn' at offset 0x328 contains 1 entries:
  Offset          Info           Type           Sym. Value    Sym. Name + Addend
000000600870  000200000006 R_X86_64_GLOB_DAT 0000000000000000 __gmon_start__ + 0

Relocation section '.rela.plt' at offset 0x340 contains 1 entries:
  Offset          Info           Type           Sym. Value    Sym. Name + Addend
000000600890  000100000007 R_X86_64_JUMP_SLO 0000000000000000
__libc_start_main@GLIBC_2.2.5 + 0

There is no dynamic relocation against fun, whose value is
resolved to zero at link-time.

readelf -r y

Relocation section '.rela.dyn' at offset 0x348 contains 2 entries:
  Offset          Info           Type           Sym. Value    Sym. Name + Addend
0000006008a0  000100000006 R_X86_64_GLOB_DAT 0000000000000000 fun + 0
0000006008a8  000300000006 R_X86_64_GLOB_DAT 0000000000000000 __gmon_start__ + 0

Relocation section '.rela.plt' at offset 0x378 contains 1 entries:
  Offset          Info           Type           Sym. Value    Sym. Name + Addend
0000006008c8  000200000007 R_X86_64_JUMP_SLO 0000000000000000
__libc_start_main@GLIBC_2.2.5 + 0
[hjl@gnu-tools-1 weak-1]$

There is dynamic relocation against fun, whose value is resolved
at runt-time. We should make it consistent, regardless if input
relocatable file is compiled with PIC or not.

I raised the issue at gABI:

https://groups.google.com/d/msg/generic-abi/Ckq19PfLxyk/eCbXPofkAQAJ

But if gABI change is incompatible with other OSes, we need to
specify it in GNU ABI.

-- 
H.J.