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: I28efa1834f58dd2a771796c58f536d249ccf2d91