Make TARGET_ARCH optional in BoardConfig

Not setting TARGET_ARCH is ok if TARGET_ARCH_SUITE is set instead. Skip
certain TARGET_ARCH-specific steps of the config:
- don't run the 'select' steps to figure out cpu flags
- don't generate dexopt config for TARGET_ARCH

Bug: 174315599
Test: lunch <product that sets TARGET_ARCH_SUITE but not TARGET_ARCH>
Change-Id: I74a9e71d0cc5c7f74d3b10b1c8bb89682c096d7c