[ARM][MC][Thumb] Revert relocation for some pc-relative fixups.

Summary:
This commit reverts https://reviews.llvm.org/D75039. Concensus appears to
be in favour of assembly-time resolution of these ADR and LDR relocations,
in line with GNU.

Reviewers: psmith

Subscribers: kristof.beyls, hiraditya, danielkiss, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78301