[ARM] PR ld/21402, only override the symbol dynamic decision on undefined weak symbol

[Jiong Wang]

>         (elf32_arm_finish_dynamic_symbol): Add sanity check.

Have you verified ARM port is immune from
https://sourceware.org/ml/binutils/2017-06/msg00412.html ?

The sanity check proven to be unnecessary and it does not contribute
to the correctness. The explanation is at:
  https://sourceware.org/ml/binutils/2017-07/msg00018.html

Regards,
Jiong