Ignore memory from elf on /memfd:

In ART, some of the maps are /memfd:/jit-cache and it triggers the warning
about unreadable elf files. Do not set the elf from memory not file
flag in this case.

Bug: 131909548

Test: New unit tests pass.
Test: No warnings dumping stacks with this change done.
Change-Id: Ifba5e65da609525ded75430da173c614f6e4801e
(cherry picked from commit 98aaf4cf086e2492796b11116e69e0ff01093663)
2 files changed
tree: 8f43b4b0eea63ec7fa2b609985c8c4874efcc000
  1. libbacktrace/
  2. libunwindstack/