[llvm-readobj/elf] - Fix the PREL31 relocation computation used for dumping arm32 unwind info (-u).

This is a part of https://bugs.llvm.org/show_bug.cgi?id=47581.

We have the following computation:
```
(1) uint64_t Location = Address & 0x7fffffff;
(2) if (Location & 0x04000000)
(3)   Location |= (uint64_t) ~0x7fffffff;
(4) return Location + Place;
```

At line 2 there is a mistype. The constant should be `0x40000000`,
not `0x04000000`, because the intention here is to sign extend the `Location`,
which is the 31 bit signed value.

Differential revision: https://reviews.llvm.org/D88407