i#1574 missing defines: define X86_32 and X86_64 inside core

Ensure X86_32 and X86_64 are defined internally, as we've started using them.
This fixes a failure in -early due to __udivdi3 and __umoddi3 turning into
imports, and a build failure in append_prepare_fcache_return().

Fixes #1574

Review-URL: https://codereview.appspot.com/191100043