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

Summary:
This commit recommits the reversion of 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. The previous
backout broke many lld tests, now fixed by Peter Smith in
61bccda9.

Reviewers: psmith

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

Tags: #llvm

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