ARM: VIXL32: Clean usage of scopes.

Use scopes in more places where they are semantically required.
There are still places where we should use scopes, but do not yet.
Those are marked with new `TODO` comments. Practically everything
works fine (see comments for details), but we eventually should fix
it.

Test: ART_USE_VIXL_ARM_BACKEND=true m test-art-host
Test: ART_USE_VIXL_ARM_BACKEND=true m test-art-target

Change-Id: I5bdc710d32f0b9476eee92b31f2ff9d874e26f63