[llvm-objdump] Avoid using mapping symbols as branch target labels

The main motivation for this change is to avoid ambiguity because
mapping symbol names may not be unique across a binary and do not allow uniquely
identifying target address. So that mapping symbols used as branch target
labels make llvm-objdump output less readable.

Another point is that mapping symbols sometimes appear in
non-allocatable sections, like debug info sections which make objdump
output even more confusing.

For example, a small AArch64 executable may contain plenty of `$d[.*]`
symbols and none of them would be useful as a label for resolving
a branch or a memory operand target address:

```
  0000000000000254 l       .note.ABI-tag	0000000000000000 $d
  00000000000008d4 l       .eh_frame            0000000000000000 $d
  0000000000000868 l       .rodata              0000000000000000 $d
  0000000000011028 l       .data                0000000000000000 $d
  0000000000010db8 l       .fini_array          0000000000000000 $d
  0000000000010db0 l       .init_array          0000000000000000 $d
  00000000000008e8 l       .eh_frame            0000000000000000 $d
  0000000000011034 l       .bss                 0000000000000000 $d
```

Note that GNU objdump doesn't use mapping symbols as branch target
labels for all targets that support such symbols (ARM, AArch64, CSKY).

Differential Revision: https://reviews.llvm.org/D139131