[ARM] halfword store hits llvm_unreachable with big-endian

Summary:
[ARM] halfword store hits llvm_unreachable with big-endian

Provide missing case in getFixupKindContainerSizeBytes().

This stops execution reaching llvm_unreachable("Unknown fixup kind!")

D83947

Reviewers: olista01, ostannard

Reviewed By: ostannard

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D83947

Change-Id: I598aa1fb51fd1c6f424c557c85d6df6d1958bc62