Add a 32-bit x86_64 arch variant

The current 32-bit configuration for generic x86_64 targets inherits some
variables (SSE4 support) from the 64-bit configuration, and overrides
the make variables used for other configurations (SSSE3). Ideally, these
would be using different variables, but until then, unify the
configuration for x86_64 targets so that everything is consistent.

Bug: 28694691
Change-Id: I47e67299d4c632e7491d7e73dc0fc6480ef08006