[PATCH] AArch64: Optimize ADD relocations that resolve to zero

[Sivan Shani]

 >> Have you looked at what other ports do?

Thank you for the suggestion. I should indeed have paid more attention, 
I have missed several relevant ld details:

1) ld provides '--no-relax'. Currently, the new optimization does not 
honor it:

```
$LD --emit-relocs --no-relax -o test.elf test.o
$OBJDUMP -dr test.elf

400120: aa0103e0  mov x0, x1
         400120: R_AARCH64_TLSLE_ADD_TPREL_HI12 tlsvar
400124: d503201f  nop
         400124: R_AARCH64_ADD_ABS_LO12_NC .data
```

'--no-relax' should preserve the original ADD instructions.

2) existing ports do not appear to require every relocation retained by 
`--emit-relocs` to remain reapplicable after optimization. There are 
cases where the original relocation is retained even though the 
instruction has been transformed. This interpretation of `--emit-relocs` 
is also discussed by MaskRay here:

https://lists.gnu.org/archive/html/bug-binutils/2023-09/msg00039.html

Other optimizations do rewrite their relocation records, so the behavior 
is not completely uniform across ports.

My current conclusion: the ADD-zero optimization do not need to be 
disabled with '--emit-relocs', as long as it honor '--no-relax'. This 
preserves the existing informational use of '--emit-relocs' while 
providing a way to request reapplicable instruction/relocation pairs.