Force enable x86inc.asm when building for x86. Previously there were
compatibility issues so a flag was added to simplify disabling this
code.

The known issues have been resolved and x86inc.asm is the preferred
abstraction layer (over x86_abi_support.asm).

BUG=b:29583530

Change-Id: Ib935e97b37ffb22d7af72ba0f04564ae6280f1fd