Revert "Make DWARF v5 the default debug information format."

This reverts commit 5e13f86f.

libabigail isn't quite ready for DWARF v5 yet, so let's revert this
temporarily.

Bug: http://b/182004844
Bug: http://b/195152239
Bug: http://b/195698910
Test: Builds
Change-Id: I4b1dd8f9e64d19f37c90b8a2433cbb302c7d5105