[Object/ELF] - Fix a position calculation expression in ELFFile<ELFT>::getEntry().
It fixes now what
1c991f907a43d7a56e82dd67a76514843841ed9a tried to fix.
(A test case failture on 32-bit Arch Linux)
On 32-bit hosts it still fails (because it truncates the `Pos` value to 32 bits).
It seems happens because of `sizeof` that returns `size_t`, which has a
different size on 32/64 bits hosts.
I've tested on a 32-bit host and verified that relocation-errors.test test and
other LLVM tools tests pass now.