[MC][ARM] Resolve some pcrel fixups at assembly time (PR44929)

MC currently does not emit these relocation types, and lld does not
handle them. Add FKF_Constant as a work-around of some ARM code after
D72197. Eventually we probably should implement these relocation types.

By Fangrui Song!

Differential revision: https://reviews.llvm.org/D72892

(cherry picked from commit 2e24219d)