Ignore R_MIPS_JALR relocations against non-function symbols

Summary:
Current versions of clang would erroneously emit this relocation not only
against functions (loaded from the GOT) but also against data symbols
(e.g. a table of function pointers). LLD was then changing this into a
branch-and-link instruction, causing the program to jump to the data
symbol at run time. I discovered this problem when attempting to boot
MIPS64 FreeBSD after updating the to the latest upstream master.

Reviewers: atanasyan, jrtc27, espindola

Reviewed By: atanasyan

Subscribers: emaste, sdardis, krytarowski, MaskRay, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70406