[llvm] Implement support for -defsym assembler option

Summary:
Changes to llvm-mc to move common logic to separate function.

Related clang patch: https://reviews.llvm.org/D26213

Reviewers: rafael, t.p.northover, colinl, echristo, rengolin

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D26214

llvm-svn: 288396