[TextAPI] Prefix all architecture enums to fix the build on i386.

Summary: This changes the Architecture enum to use a prefix (AK_) to prevent the
preprocessor from replacing i386 with 1 when building llvm/clang for i386.

Reviewers: steven_wu, lhames, mstorsjo

Reviewed By: mstorsjo

Subscribers: hiraditya, jkorous, dexonsmith, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60241

llvm-svn: 357733