Fix R_AARCH64_MOVW_UABS_G3 relocation

Summary: The relocation is missing mask so an address that has non-zero bits in 47:43 may overwrite the register number. (Frequently shows up as target register changed to `xzr`....)

Reviewers: t.p.northover, lhames

Subscribers: davide, aemerson, rengolin, llvm-commits

Differential Revision: https://reviews.llvm.org/D27609

llvm-svn: 289880