Revert^2 "ART: Fix breaking changes from recent VIXL update."

This reverts commit 9477047a.

Recent AOSP/VIXL update exposed the problem of loading two separate VIXL
libraries libvixl-arm and libvixl-arm64 in one process. This revert
fixed this issue by building & loading only one libvixl from now on.

Test: test-art-target, test-art-host
Test: art-gtest-asan

Change-Id: I6d59c29c0b705320d5c855017b987fc7060c32cc