Use unified libvixl(d) instead of per-supported-arch libvixl(d)-arm(64).

Using a unified libvixl(s) saves space by sharing code that was common
to libvixl(d)-arm and libvixl(d)-arm64 and also addresses multiple
definitions issues that would cause ODR violations.

Test: 32-bit-only device boot test
Test: 32/64-bit device boot test
Change-Id: I1d67c43897e08bc0f5743eb038fe574ce2fb54f3