gas: add support for DWARF-5 and experimental two-level line number tables.

This change is forward-port of commit a7c7bcafd2add7ecf8ea2ad7d3d77cf38d46c195
  - https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=a7c7bcafd2add7ecf8ea2ad7d3d77cf38d46c195
from upstream branch users/ccoutant/two-level-line-150331
  - https://sourceware.org/git/?p=binutils-gdb.git;a=shortlog;h=refs/heads/users/ccoutant/two-level-line-150331
to binutils-2_27-branch.

Change-Id: I070d675b2403325fe3087be71b158ff765a8aab1
3 files changed