MC: Implement support for new .addrsig and .addrsig_sym directives.

Part of the address-significance tables proposal:
http://lists.llvm.org/pipermail/llvm-dev/2018-May/123514.html

Differential Revision: https://reviews.llvm.org/D47744

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@337328 91177308-0d34-0410-b5e6-96231b3b80d8
16 files changed