Recognize -2 as a tombstone value in .debug_line

Commit a8caed5d handled the tombstone
value -1 used by lld (https://reviews.llvm.org/D81784).  The
referenced lld commit also uses the tombstone value -2 for
pre-DWARF-v5
(https://github.com/llvm/llvm-project/commit/e618ccbf431f6730edb6d1467a127c3a52fd57f7).

If not handled, -2 breaks the pc step range calculation and triggers
the assertion:

  gdb/infrun.c:2794: internal-error: resume_1: Assertion
  `pc_in_thread_step_range (pc, tp)' failed.

This commit adds -2 tombstone value and handles it in the same way as -1.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31727


Cherry-picked from e814012b
Approved-By: Tom Tromey <tom@tromey.com>